BIOLOGY 

LIBRARY 

6 


I 


VERTEBRATE  ZOOLOGY 


THE  MACMILLAN  COMPANY 

NEW  YORK   •    BOSTON   •    CHICAGO    •   DALLAS 
ATLANTA    •   SAN  FRANCISCO 

MACMILLAN  &  CO.,  LIMITED 

LONDON    •  BOMBAY   •  CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  LTD. 

TORONTO 


VERTEBRATE  ZOOLOGY- 


BY 

HORATIO  HACKETT  NEWMAN,  Pn.D, 

/j 

PROFESSOR  OF  ZOOLOGY  AND  EMBRYOLOGY  IN  THE 
UNIVERSITY  OF  CHICAGO 


fork 
THE  MACMILLAN  COMPANY 

1926 

All  rights  reserved 


COPYRIGHT,  1920, 
BY   THE    MACMILLAN   COMPANY. 


Set  up  and  electrotyped.     Published  February,  1920.    Reprinted 
February,  1926. 


PRINTED  IN  THE   UNITED   STATES   OF  AMERICA   BY 
BERWICK  &  SMITH  CO. 


THIS  VOLUME  IS  DEDICATED 

TO  MY  FATHER 
ALBERT  HENRY  NEWMAN. 


602168 


PREFACE 

This  volume  is  intended  for  use  as  a  text-book  in  college  courses  in 
vertebrate  zoology  such  as  are  required  of  premedical  students  and 
others  who  have  had  a  course  in  general  zoology.  The  aim  of  the  book 
is  to  present  those  aspects  of  the  subject  which  are  not  adequately 
brought  out  by  laboratory  work  in  comparative  anatomy.  It  is  taken 
for  granted  that  the  student  who  uses  this  as  a  text  in  connection 
with  the  lecture  and  recitational  part  of  a  course,  shall  also  pursue  a 
laboratory  course  in  comparative  anatomy,  using  a  laboratory  man- 
ual. It  is  also  believed  advisable  to  use  as  laboratory  references  the 
various  text-books  on  comparative  anatomy. 

The  book  is  avowedly  dynamic  in  tone,  emphasizing  the  physio- 
logical, developmental,  phylogenetic,  and  ecological  aspects  of  ver- 
tebrates. Structural  features  must  of  course  be  dealt  with  extensively, 
but  purely  anatomical  details  are  as  a  rule  subordinated  to  physio- 
logic and  evolutionary  considerations. 

The  vertebrates  are,  moreover,  viewed  not  merely  as  a  group  of  ani- 
mals belonging  to  the  present,  but,  historically,  as  a  very  ancient  as- 
semblage of  related  forms,  that  arose  from  simple  beginnings  many 
millions  of  years  ago  and  have  passed  through  many  vicissitudes  in- 
volved in  the  mighty  world  changes  of  ancient  times.  Hence  more 
than  the  usual  attention  is  given  to  earlier  chapters  in  the  ancestral 
history  of  the  vertebrate  classes,  chapters  that  are  often  of  more  dra- 
matic interest  than  those  of  the  present  and  that  give  to  the  student 
a  new  conception  of  the  significance  of  modern  end-products  of  evo- 
lution which,  in  themselves,  are  often  relatively  unattractive  and  de- 
void of  interest. 

The  writer  has  for  some  years  been  much  impressed  with  the  far- 
reaching  applicability  to  problems  of  animal  morphology,  of  the  ax- 
ial gradient  conception  of  his  colleague,  Professor  Child,  and  one  of 
the  features  of  the  present  book  is  the  attempt  to  interpret  verte- 
brate structures  in  terms  of  this  conception.  In  some  cases  it  is 
probable  that  the  theory  has  been  stretched  beyond  the  limits  its 
author  would  consider  justified;  hence  the  present  writer  takes  en- 

vii 


viii  PREFACE 

tire  responsibility  for  the  applications  of   Child's  theory  brought 
out  in  this  volume. 

The  axial  gradient  conceptions  have  appeared  to  the  writer  to  be 
strictly  in  accord  with  the  principles  of  racial  senescence  as  presented 
by  H.  F.  Osborn  and  R.  S.  Lull  in  their  recent  volumes  on  evolu- 
tion. The  present  volume  has  much  to  say  about  adaptive  radiation 
and  the  various  degenerative  and  senescent  conditions  so  common 
among  vertebrates.  The  writer  has  freely  made  use  of  the  material 
found  in  Osborn's  and  in  Lull's  books.  Many  figures  have  been  bor- 
rowed from  both  authors,  for  which  the  writer  is  deeply  indebted. 

Much  of  the  data  used  has  been  taken  from  the  several  volumes  on 
vertebrates  of  the  Cambridge  Natural  History,  and  from  the  various 
comparative  anatomies  such  as  those  of  Wiedersheim,  of  Kingsley, 
and  of  Wilder.  Kellicott's  "  Chordate  Embryology  "  has  been  found 
excellent  for  much  of  the  embryological  data.  A  list  of  about  a  hun- 
dred references  to  which  the  writer  has  had  access  is  presented  in  an 
appendix. 

The  illustrations  have  been  borrowed  to  a  considerable  extent  from 
Macmillan  Company  publications,  but  the  great  majority  of  the  fig- 
ures have  been  redrawn  by  Mr.  Kenji  Toda  to  whom  I  herewith  ex- 
press my  hearty  thanks.  Many  of  the  figures  are  made  up  of  several 
related  illustrations  arranged  in  such  fashion  that  they  readily  may 
be  put  into  chart  form.  In  our  own  laboratory  we  are  already  using 
a  considerable  proportion  of  these  compound  figures  as  charts.  The 
sources  of  these  redrawn  figures  are  various  and  the  author  wishes  to 
acknowledge  with  thanks  the  courtesy  of  those  who  have  permitted 
their  originals  to  be  thus  modified  and  used.  The  greatest  care  has 
been  taken  properly  to  acknowledge  the  source  of  each  borrowed  il- 
lustration. If  in  any  case  the  figure  has  been  incorrectly  attributed 
to  an  author,  information  regarding  the  error  will  be  gratefully 
received. 

The  author  is  not  unaware  of  the  shortcomings  of  the  present  book. 
Some  critics  will  doubtless  feel  that  much  valuable,  and  to  their  minds 
necessary,  data  has  been  omitted.  Others  will  probably  believe  that 
much  that  has  been  included,  especially  in  connection  with  the  notes  on 
the  natural  history  of  certain  types,  might  better  have  been  omitted. 
But  the  selection  of  what  to  present  and  what  to  omit  has  been  care- 
fully canvassed  and  what  appears  to  be  a  workable  compromise  has 
been  decided  upon.  The  teacher  may  readily  omit  what  he  feels  to  be 


PREFACE  ix 

superfluous,  and  will  be  glad  to  supply  the  data  that  he  feels  should 
be  included. 

Doubtless,  errors  of  various  kinds  have  crept  into  the  text  in  spite 
of  our  scrupulous  efforts  to  avoid  them.  Although  the  writer  has  been 
indebted  to  several  competent  readers  of  his  manuscript,  he  holds 
himself  entirely  responsible  for  the  book  with  all  of  its  defects  and 
whatever  virtues  it  may  have.  He  will  be  grateful  for  criticism  and 
correction  on  the  part  of  his  readers. 

H.  H.  NEWMAN. 
Chicago,  111. 

May  1,  1919. 


TABLE  OF  CONTENTS 

CHAPTER  I.    PRINCIPLES  OF  VERTEBRATE  MORPHOLOGY 

The  fundamental  architectural  plan  of  vertebrates 1-8 

A  physiological  interpretation  of  the  fundamental  structural  plan  of 

vertebrates 8-12 

Generalized,  specialized,  senescent,  and  retarded  (paedogenetic) 

types  of  vertebrates 12-22 

Vertebrate  phylogeny 22-33 

CHAPTER  II.    THE  PHYLUM  CHORDATA  ' 

Cephalochordata 33^8 

Urochordata 48-60 

Hemichordata. 60-68 

Craniata  (Vertebrates) 68-70 

CHAPTER  III.    THE  ORIGIN  AND  EVOLUTION  OF  THE  VERTEBRATES 

The  Amphioxus  Theory 72-75 

The  Annelid  Theory 77-79 

The  Arthropod  Theory .79-82 

Minor  Theories 82-86 

CHAPTER  IV.    CYCLOSTOMATA 

Myxinoidea 89-91 

Petromyzontia 91-96 

CHAPTER  V.    PISCES  (FISHES) 

Introduction  to  Pisces 97-112 

Elasmobranchii 112-127 

Teleostomi m. *, 127-153 

Dipneusti 153-159 

Generalized  and  specialized  types  of  fishes  and  the  axial  gradient 

theory  of  structural  relations 159-163 

Eggs,  reproduction,  and  breeding  habits  of  fishes 163-168 

Appendix  to  fishes  (Ostrachodermi,  etc.) . 168-172 

xi 


xii  TABLE  OF  CONTENTS 

CHAPTER  VI.   AMPHIBIA  PAGES 

Introductory 173-179 

Extinct  Amphibia 180-183 

Present-day  Amphibia 183-203 

The  development  of  the  frog 203-209 

CHAPTER  VII.    REPTILIA 

Introductory 210-213 

Extinct  reptiles 213-224 

Modern  reptiles 225-259 

CHAPTER  VIII.   AVES  (BIRDS) 

Introductory 260-276 

Extinct  birds 276-281 

Birds  of  to-day 281-312 

Ratitse 281-288 

Carinatse 288-312 

Development  of  birds .314-323 

CHAPTER  IX.    MAMMALIA 

Introductory • 324-336 

Extinct  mammals 336-345 

Mammals  of  the  present 345-359 

Prototheria 347-352 

Eutheria  (Didelphia) ; 352-359 

CHAPTER  X.    MAMMALIA  (CONTINUED)  PLACENTAL  MAMMALS 

Unguiculata 360-363 

Dermoptera 363 

Chiroptera 363-364 

Carnivora 364-371 

Rodentia 371-373 

Edentata 373-377 

Tubulidentata 377-378 

Primates 378-388 

Artiodactyla ' 389-392 

Perissidactyla 392-396 

Sirenia 396-398 

Hyracoidea 398-400 


TABLE  OF  CONTENTS  xiii 

PAGES 

Odontoceti 400-403 

Mystacoceti 403-404 

The  development  of  mammals 404-411 

PARTIAL  LIST  OP  LITERATURE 413-415 

INDEX ,  .  .417-432 


VERTEBRATE  ZOOLOGY 


VERTEBRATE  ZOOLOGY 

CHAPTER  I 
PRINCIPLES  OF  VERTEBRATE  MORPHOLOGY 

Definition. — The  vertebrates  may  be  defined  as  animals  having: — 
pronounced  antero-posterior,  dorso-ventral,  and  bilateral  axes;  inter- 
nal metameric  segmentation,  especially  of  the  mesoblast;  a  central 
nervous  system  dorsal  in  position  and  tubular  in  structure,  with  a  well- 
defined  central  canal  or  neurocoel;  a  well-defined  head,  characterized 
by  highly  specialized  sense  organs  and  by  a  concentration  of  nervous 
tissue  into  a  complex  brain ;/i^_aJimentajy  tract  opening  by  anterior 
jnouth _ andjposterior  anus,  and  provided  with  paired  pharyngeal 
clefts;  a  notochord^erived  from  the  primitive  endoderm  and  situated 
Between  the  central  nervous  system  and  the  alimentary  tract;  an  open 
ccelom,  at  first  segmented,  but  later  the  segmental  ccelomic  cavities 
unite  to  form  the  large  pericardial,  peritoneal,  aricj,  in  the  mammals, 
thoracic  cavities  j  a 'closed  circulatory  system  quite  distinct  from  the 
cu'lom;  a  post-anal  prolongation  of  the  body  into  a  metamerically 
segmented  tail,  without  coelomic  cavity;  usually  paired  appendages, 
pectoral  and  pelvic.  These  characters  and  a  few  others  serve  to  mark 
off  the  vertebrates  quite  sharply  from  all  other  groups.  Several  of  the 
most  fundamental  of  these  characteristics  must  now  be  discussed. 
The  diagrams  on  the  following  page  (Fig.  1)  illustrate  most  of  these 
characters. 

THE  FUNDAMENTAL  ARCHITECTURAL  PLAN  OF  VERTEBRATES 
THE  THREE  MORPHOLOGICAL  AXES 

The  Axis  of  Polarity  (Primary  Axis). — A  typical  vertebrate  has  an 
elongated  form,  with  head  and  tail  ends  clearly  defined.  An  imagi- 
nary line  drawn  from  the  extreme  anterior  to  the  extreme  posterior 
end  indicates  the  primary  structural  and  functional  axis  of  the  body, 
which  is  designated  the  antero-posterior  or  apico-basal  axis.  The  or- 

1 


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PRINCIPLES  OF  VERTEBRATE   MORPHOLOGY  3 

gans  of  highest  dynamic  activity  and  most  pronounced  sensitivity  are  at 
the  apical  or  anterior  end  and  the  organs  of  lowest  dynamic  activity  and 
least  sensitivity  are  at  the  basal  or  posterior  end  of  this  axis.  Between 
these  extremes,  or  opposite  poles,  of  the  axis  the  remaining  organs 
or  functions  are  arranged,  at  least  primitively,  in  a  graded  series  of 
diminishing  dynamic  activity  and  sensitivity.  These  geometrical 
relations  serve  as  an  index  of  an  inherent  spatial  orderliness  in  the 
arrangement  of  the  functions  with  reference  to  one  another,  and 
demonstrate  that  the  organism  is  based  on  a  single  plan — is  a  coher- 
ent entity. 

From  a  purely  physiological  point  of  view  this  gradient  represents 
a  linear  series  of  functions,  ranging  from  dominant  or  controlling  func- 
tions to  subordinate  or  controlled  functions,  a  series  which,  broadly 
speaking,  runs  somewhat  as  follows : — olfactory  and  visual,  the  most 
anterior  and  dominant  functions,  entirely  sensory  in  character;  motor 
functions  associated  with  movement  of  the  eyes,  and  motor  centers 
for  most  voluntary  functions;  sensory  and  motor  activities  associated 
with  feeding,  including  the  sense  of  taste,  and  the  motor  activities  of 
jaws  and  tongue;  sensory  and  motor  activities  associated  with  hearing 
and  equilibrium;  the  active  functions  of  respiration  and  circulation, 
which  are  closely  correlated;  the  most  anterior  locomotor  functions, 
associated  with  the  pectoral  appendages;  the  most  active  phases  of 
the  alimentary  or  digestive  functions,  associated  with  the  stomach 
and  the  larger  glands;  the  excretory  and  lower  alimentary  functions, 
associated  with  the  kidneys,  the  lower  intestine  and  rectum;  the  re- 
productive functions,  associated  with  ovaries  and  testes,  their  acces- 
sory ducts  and  copulatory  organs;  the  functions  of  the  tail  or  post-anal 
body,  which  may  be  considered  as  a  developmental  afterthought  and 
as  more  or  less  beyond  the  limits  of  the  original  primary  axis.  The 
tail  has  a  gradient  of  its  own  and  does  not  belong  to  the  primary  gra- 
dient. This  is  only  a  rough  outline  of  the  real  physiological  gradient, 
but  is  clear  enough  for  our  purposes.  The  true  gradient  no  longer 
exists  in  modern  vertebrates  because  there  has  been  a  great  deal  of 
secondary  concentration  at  various  levels,  especially  at  the  anterior 
end,  where  the  original  metameric  arrangement  of  the  functional 
series  has  been  profoundly  disturbed  and  distorted.  The  primary 
gradient  is  further  obscured  by  the  fact  that  various  systems  of  or- 
gans, such  as  the  heart  and  blood  vessels,  the  brain,  the  alimentary 
tract,  etc.,  have  developed  secondary  axes  of  functional  activity  and 


VERTEBRATE  ZOOLOGY 


structural  differentiation  that  are  largely  independent  of  the  primary 
axis  and  have  little  reference  to  the  polarity  of  the  body  as  a  whole. 
In  the  embryo,  however,  the  axis  is  less  distorted  than  in  the  adult. 
The  Dorso- Ventral  Axis  (Secondary  Axis). — If  we  take  a  cross 
section  at  any  level  of  the  primary  axis,  we  at  once  perceive  that  a 

line  drawn  from  the 
mid-dorsal  to  the 
mid-ventral  line  rep- 
resents another  main 
architectural  axis.  In 
the  diagram  (Fig.  2) 
it  will  be  noted  that 
the  central  nervous 
system  occupies  the 
high  point  or  apical 
end  of  the  axis  and 
that,  at  this  level, 
one  loop  of  the  intes- 
tine occupies  the  ven- 
tral or  basal  point  of 
the  axis.  The  grad- 
ient of  functions  in 
between  the  two  poles 
has  been  so  decidedly 
disturbed  by  lateral 
foldings  and  second- 
ary displacements, 
that  any  attempt  to 
construct  a  list  of 
graded  functions 
FIG.  2. — Diagrammatic  transverse  section  of  a  verte-  would  be  futile.  We 
brate  to  illustrate  the  dorso- ventral  (secondary)  and  know  jn  general  how- 
bilateral  (tertiary)  axes  of  organization,  al,  alimentary  ' 
tract;  ao,  aorta ',d.m,  dorsal  mesentery;  my,  myotome;  ever,  that  the  dorsal 
nc,  notochord;  n,  nephrotome;  o,  omentum;  sc,  spinal  side  is  the  dominant 
chord;  v.m,  ventral  mesentery.  (Modified  after  t  of  the  axig  and 

Kingsley.)  f.  ,     .  , 

that,   during   embry- 
onic development,  the  various  functions  differentiate  almost  exactly 
in  the  order  of  their  relative  dynamic  activity. 
The  Bilateral  Axis  (Tertiary  Axis). — The  dorso-ventral  axis  divides 


PRINCIPLES  OF  VERTEBRATE   MORPHOLOGY  5 

any  level  of  the  antero-posterior  axis  into  two  mirror-image  halves 
(Fig.  2),  so  that  each  side  is  the  reversed  counterpart  of  the  other.  It 
would  appear  then  that  the  bilateral  axis  is  a  necessary  consequence  of 
the  dorso-ventral  axis.  The  axis  has  a  single  median  or  apical  point 
and  two  lateral  or  basal  points.  Any  vertical  level  in  the  dorso-ven- 
tral axis  may  constitute  the  apical  point  of  a  double  bilateral  axis. 
A  good  example  of  this  type  of  axial  organization  is  seen  in  connection 
with  the  differentiation  of  the  segmented  mesoblast  of  the  chick 
(Fig.  3).  The  median  dorsal  part  of  the  somite  forms  the  myotome 
(segmental  voluntary  muscles),  the  most  highly  dynamic  of  all  meso- 
dermal  structures;  next  comes  the  derma  tome  which  forms  the  deeper 
skin  and  many  complex  sensory  and  glandular  structures,  etc.;  next 
comes  the  nephrotome  or  primordium  of  the  excretory  system;  next, 


Coel. 


FIG.  3. — Transverse  section  across  the  primary  axis  of  a  10  somite  chick  em- 
bryo, to  illustrate  the  bilateral  (tertiary)  axis  of  vertebrate  organization.  Ao, 
aorta;  Coel,  ccelom;  ect,  ectoderm;  ent,  entoderm;  nch,  notochord;  N.F.,  neural 
furrow;  N.  ph,  nephrotome;  s,  somite;  sow.  Mes,  somato-pleure;  spl.  Mes,  splan- 
chopleure.  (From  Lillie's  "  Development  of  the  Chick  "  [Henry  Holt  and  Co].) 

but  only  at  posterior  levels,  the  gonatome  or  primordium  of  the  re- 
productive system;  next,  the  embryonic  ccelom,  from  which  are  de- 
rived mainly  such  passive  structures  as  the  peritoneal  lining  of  the 
body  cavities  and  the  mesenteries;  and  finally,  the  extra-embryonic 
body  cavity,  which  has  to  do  primarily  with  the  formation  of  embry- 
onic membranes  that  serve  as  protective  and  respiratory  organs  dur- 
ing the  life  of  the  embryo,  but  play  no  role  in  the  organization  of  the 
adult. 

It  should  be  understood  that  the  appendages  are,  as  the  name  indi- 
cates, truly  added  structures  and  are  not  to  be  considered  as  part  of 
the  original  bilateral  gradient.  Each  appendage  has  its  own  three 
gradients,  with  the  free  end  representing  the  apical  end  and  the  fixed 
end,  the  basal.  It  need  hardly  be  said  that  the  main  specializations 
of  the  appendages  take  place  at  the  apical  or  free  end,  while  the  basal 
parts  remain  comparatively  conservative  and  undifferentiated. 


6  VERTEBRATE  ZOOLOGY 

METAMERISM 

The  segmental  organization,  metameric  structure,  is  secondary  in 
importance  only  to  the  primary  axiate  organization  and  is  undoubt- 
edly one  of  the  developmental  consequences  of  the  latter.  The  ver- 
tebrates constitute  but  one  of  three  great  metameric  groups,  the  other 
two  being  the  annelids  and  the  arthropods.  Annelids,  the  lowest  of 
metameric  types,  show  metamerism  in  its  most  generalized  condition. 
In  them  segmentation  involves  equally  both  internal  and  external 
structures,  and  there  is  little  or  no  obliteration  of  primary  metamerism 
by  fusion  of  groups  of  contiguous  metameres  into  larger  regions. 
In  the  arthropods  the  external  metamerism  to  a  large  extent  retains  its 
primitive  condition,  especially  in  regions  back  of  the  head  or  cephalo- 
thojax,  but  the  primitive  internal  metamerism  is  obscured  through 
the  almost  complete  obliteration  of  the  ccelom  by  the  system  of  ve- 
nous sinuses. 

Metamerism  in  the  vertebrates,  especially  in  the  higher  groups, 
is  confined  largely  to  certain  internal  structures  and  is  scarcely  at  all 
expressed  in  external  characters.  Even  the  internal  metamerism  is 
to  a  large  extent  obscured  by  the  compacting  of  the  anterior  meta- 
meres into  a  head,  and  by  the  fusion  of  the  original  segmental  body 
cavities  into  a  few  large  compound  ccelomic  chambers.  The  segmen- 
tation is  best  expressed  in  the  structures  derived  from  the  embryonic 
mesoblast:  the  myotomes,  the  nephric  tubules,  the  vertebral  units, 
and  their  skeletal  accessories.  The  central  nervous  system  also  re- 
tains its  primitive  metameric  segmentation  in  post-cephalic  regions, 
but  in  the  cephalic  region  the  metameric  elements  are  very  difficult 
to  make  out.  There  are  believed  to  be  primitively  five  (possibly  six) 
metameres,  three  or  four  in  front  of  the  otic  vesicle  and  one  behind  it. 

Back  of  the  head,  however,  the  vertebrate  is  composed  of  a  linear 
series  of  body  segments  or  metameres,  each  of  which  has,  theoreti- 
cally, its  own  complete  set  of  body  organs.  There  is,  however,  never 
an  ideal  or  complete  development  of  all  the  characteristic  segmental 
organs  in  any  one  metamere.  In  the  anterior  metameres  the  organs 
of  lowest  dynamic  activity,  such  as  those  of  excretion  and  reproduc- 
tion, do  not  appear;  and  in  the  posterior  metameres  the  organs  of  high- 
est dynamic  activity,  such  as  the  organs  of  special  sense  and  the 
higher  coordinative  faculties,  do  not  appear.  And  there  is  a  graded 
series  between  these  extremes. 


PRINCIPLES  OF  VERTEBRATE  MORPHOLOGY  7 

As  a  rule,  a  series  of  two  to  ten  or  more  metameres  unites  into  a 
functional  region  and  a  concentration  of  these  metameres  occurs  that 
makes  it  difficult  to  discover  the  original  segmental  conditions  in- 
volved. Even  the  most  primitive  vertebrates  exhibit  wide  departures 
from  the  primitive  metameric  plan  seen  in  some  of  the  lower  anne- 
lids, and  these  modifications  of  the  generalized  segmental  arrange- 
ment become  progressively  more  pronounced  from  the  lowest  to 
the  highest  vertebrate  classes. 

CEPHALIZATION 

One  of  the  most  significant  advances  over  ancestral  conditions 
that  the  vertebrates  have  to  their  credit  has  to  do  with  the  evolution 
of  intelligence,  and  this  is  intimately  bound  up  with  the  pronounced 
specialization  of  the  anterior  metameres  into  a  head,  which  is  princi- 
pally a  brain  and  a  group  of  sense  organs.  This  foreshortening,  con- 
densation, and  specialization  of  the  anterior  metameres  has  been 
called  cephalization.  The  course  of  evolution  from  the  lowest  to  the 
highest  vertebrates  has  been  one  of  more  and  more  pronounced 
cephalization  involving  a  progressively  larger  number  of  metameres, 
an  increased  dominance  of  the  head  over  the  rest  of  the  body,  and 
closer  integration  of  all  the  functions.  The  climax  of  the  process  of 
cephalization  is  Man,  who  is  essentially  a  dominating  intelligence 
and  a  set  of  accessory  organs.  Various  attempts  have  been  made  to 
decipher  the  highly  modified  metamerism  of  the  vertebrate  head. 
One  of  these  attempts  gave  rise  to  the  classic  "  Vertebral  Theory  " 
of  the  skull  by  Goethe  and  by  Oken.  It  was  thought  that  the  skull 
was  made  up  of  a  modified  series  of  vertebral  units  and  that  an  analysis 
of  these  elements  would  give  the  number  of  metameres.  This  theory 
was  proven  by  Huxley  to  be  untenable.  The  cranial  nerves  have  been 
taken  as  evidences  of  metamerism,  but  the  old  arrangement  of  ten 
cranial  nerves  is  no  longer  taken  to  indicate  ten  metameres,  since  some 
of  these  nerves  are  the  motor  and  others  the  sensory  components  of 
single  metameric  divisions  of  the  neuron.  Perhaps  the  most  reliable 
index  of  the  number  of  metameres  in  the  head  is  seen  in  the  mesoblas- 
tic  somites  of  some  of  the  fishes  and  cyclostomes.  There  are  but  three 
somites  in  front  of  the  auditory  capsule,  which  forms  a  convenient 
landmark  in  that  it  seems  to  mark  off  the  original  head  from  the  post- 
cephalic  region.  Possibly  then  the  primitive  craniate  had  a  head 
of  three  metameres,  a  number  characteristic  of  the  larva  of  Balano- 


8  VERTEBRATE  ZOOLOGY 

glossus,  and  possibly  that  of  the  echinoderms.  The  most  characterise 
tic  stage  in  certain  annelid  larvae  has  also  three  metameres.  The 
higher  vertebrates  have  in  the  process  of  their  evolution  gradually 
appropriated  to  the  head,  one  after  the  other,  the  adjacent  body 
metameres. 

The  Backward  Retreat  of  the  Lower  Functions. — Accompanying 
the  concentration  of  the  chief  nervous  elements  in  the  head  there  is 
a  progressive  series  of  changes  in  the  opposite  direction,  consisting  of 
a  steady  retreat  toward  the  posterior  end  of  the  respiratory,  digestive, 
excretory,  and  reproductive  functions.  These  functions  are,  in  the 
more  primitive  vertebrates,  present  in  the  metameres  just  back  of  the 
head,  but  in  the  higher  forms  they  are  steadily  pushed  back  into  the 
posterior  metameres  by  becoming  atrophied  in  the  anterior  metameres 
and  by  the  development  of  new  sets  of  more  or  less  equivalent 
organs  further  back.  An  excellent  example  of  this  type  of  process  is 
seen  in  the  evolution  of  the  kidneys.  In  the  most  primitive  verte- 
brates the  embryonic  kidney  is  located  well  toward  the  anterior  part 
of  the  ccelom,  and  in  the  hag-fishes  this  anterior  kidney,  or  pronephros, 
functions  in  the  adult.  In  vertebrates  of  the  fish  and  amphibian 
grades  the  kidney  of  the  adult  is  a  mesonephros,  located  farther  back 
in  the  body  cavity;  while  in  the  land  vertebrates  the  functional  kid- 
ney is  a  metanephros,  situated  still  farther  toward  the  posterior  end. 

The  farther  these  organs  of  lower  dynamic  activity  retreat  from 
the  dominant  anterior  end  the  more  readily  they  appear  to  dif- 
ferentiate and  the  more  specialized  and  condensed  they  become,  just 
as  though  they  were  steadily  growing  out  from  under  an  influence 
that  tends  to  suppress  their  full  developmental  possibilities.  The  evo- 
lution of  the  urogenital  organs  of  the  vertebrates  affords  an  interest- 
ing example  of  this.  In  the  lowest  vertebrates  the  gonads  and  their 
ducts  are  simple,  are  located  far  forward  in  the  body  cavity,  and  are 
to  a  large  extent  separate  from  the  urinary  elements.  In  the  highest 
vertebrates,  however,  these  organs  have  retreated  far  to  the  poste- 
rior end  of  the  primary  axis  and  have  been  intimately  involved  with 
the  urinary  system. 

A    PHYSIOLOGICAL    INTERPRETATION    OF   THE    FUNDAMENTAL 
STRUCTURAL  PLAN  OF  VERTEBRATES 

Professor  C.  M.  Child  has  developed  what  appears  to  the  writer 
to  be  the  most  scientific  and  far-reaching  dynamic  interpretation  of 


PRINCIPLES  OF  VERTEBRATE   MORPHOLOGY  9 

animal  structure  that  has  yet  appeared.  His  " axial  gradient  theory'' 
is  a  statement  in  energistic  terms  of  the  axiate  organization  of  animals 
and  of  the  relations  of  dominance  and  subordination  that  are  so  ob- 
vious in  such  animals  as  the  vertebrates.  He  has  shown  experimen- 
tally "that  the  apical  or  anterior  region  is  primarily  the  region  of 
greatest  dynamic  or  metabolic  activity  in  the  individual.  The  apical 
end  becomes  the  most  highly  specialized  and  differentiated  region  of 
the  body,  and  in  those  forms  which  possess  a  central  nervous  system, 
the  cephalic  ganglion  or  brain  and  the  chief  sense  organs  usually 
arise  in  this  region;  ...  in  other  words  it  becomes  the  head,  and  in 
motile  forms  usually  precedes  in  locomotion."  "The  basal  or  post- 
erior region,  on  the  other  hand,  is  primarily  the  least  active  region  and 
in  motile  forms  its  activity  is  more  or  less  under  the  control  of  the 
apical  or  anterior  region." 

THE  AXIAL  GRADIENT  IN  DEVELOPMENT 

There  is  a  remarkable  parallelism  between  the  orderly  spatial 
arrangement  of  structures  and  functions  down  the  polar  axis  and  the 
orderly  sequence  in  time  of  these  structures  during  the  develop- 
ment of  the  individual  from  the  egg.  The  first  structures  to  differ- 
entiate are  those  of  the  head;  in  fact  the  early  vertebrate  embryo  is 
nearly  all  head  and  the  body  grows  out  from  it  as  though  it  were  a 
mere  axial  outgrowth  of  the  head.  The  last  part  of  the  body  to  com- 
plete its  differentiation  is  the  tail  or  basal  part  of  the  axis  of  polarity. 
There  is  thus  an  important  relation  between  the  degree  of  dominance 
and  the  developmental  age  of  the  various  levels  along  the  axis.  The 
more  anterior  part  always  differentiates  before  the  more  posterior 
part  and  dominates  it  at  least  for  a  time.  Secondary  alterations  in  the 
relationships  of  dominance  and  subordination  are  of  frequent  occur- 
rence and  result  from  functional  adjustments  and  through  the  action 
of  external  factors.  The  same  parallelism  between  spatial  arrange- 
ment and  sequence  in  time  exists  for  the  dorso-ventral  axis.  In  verte- 
brates, the  first  structures  to  differentiate  are  those  in  the  medullary 
plate  region;  in  fact,  in  the  higher  vertebrates,  the  early  embryo  con- 
sists almost  entirely  of  the  anterior  median  dorsal  region,  which  is 
destined  to  form  the  central  nervous  system  of  the  head.  The  last 
structures  fully  to  differentiate  are  the  reproductive  structures  which, 
functionally  at  least,  belong  to  the  basal  part  of  the  gradient.  Struc- 
tures also  develop  mesio-laterally,  differentiating  first  in  the  middle 


10  VERTEBRATE  ZOOLOGY 

and  proceeding  laterally,  so  that,  in  the  case  of  such  structures  as  the 
paired  appendages,  the  last  parts  to  develop  are  the  terminal  elements. 

The  Nervous  System  as  a  Mechanism  for  Maintaining  the 
Relations  of  Dominance  and  Subordination  in  the  Organism.— 
The  method  of  maintaining  the  dominance  of  the  apical  parts  over 
basal  parts  is  the  method  of  conduction  over  nerve  paths.  There 
seems  to  be  little  doubt  now  that  communications  between  an  apical 
part  and  a  basal  part  are  electroid  in  character,  so  that  changes  in  the 
chemical  state  of  the  apical  part  are  transmitted  speedily  to  the  basal 
part  and  excite  it  to  functional  activity  or  inhibit  its  action,  according 
to  the  character  of  the  apical  change.  Thus  a  stimulated  condition 
of  a  particular  segment  of  the  central  nervous  system  involves  a 
change  of  electric  potential  between  it  and  the  basal  organ  with  which 
it  is  connected.  The  reaction  of  the  basal  part  is  in  the  nature  of  an 
adjustment  to  meet  the  change  in  the  apical  part  so  as  to  bring  about 
a  return  to  equilibrium  between  the  two  terminal  points  of  the  system. 
Thus,  throughout  the  whole  body  there  is  a  very  intricate  system 
of  controlling  and  controlled  parts,  depending  on  the  relative  inten- 
sity or  relative  rate  of  metabolism  of  the  various  parts.  It  has  been 
shown  by  the  most  exact  types  of  apparatus,  e.  g.,  the  Tashiro  biom- 
eter,  which  measures  exceedingly  minute  differences  in  carbon  dioxide 
production  in  small  objects  such  as  nerves,  that  the  apical  part  of  a 
nerve  has  a  higher  rate  of  metabolism  than  the  basal  part. 

Relative  Susceptibility  to  Inhibiting  Agents  of  the  Apical 
and  Basal  Parts  of  the  Axes.  Child  has  demonstrated  differ- 
ences in  rate  of  metabolism  between  apical  and  basal  parts  of 
an  organism  by  the  so-called  "susceptibility  method."  He  finds 
that  apical  parts  are  more  susceptible  than  basal  parts  to  agents 
that  retard  metabolic  activity  (such,  as  anaesthetics,  potassium 
cyanide,  etc.).  If  lethal  concentrations  are  used,  the  apical  part 
dies  first  and  there  is  a  progression  of  death  changes  down  the 
axes  until  the  last  parts  to  retain  life  are  the  basal  parts.  These 
experiments  demonstrate  that  the  axial  gradient  is  essentially  a 
gradient  in  the  rate  or  intensity  of  metabolic  activity.  From  this  we 
may  conclude  that  the  relationship  of  functional  dominance  and  sub- 
ordination is  one  depending  on  the  relative  rates  of  metabolism,  the 
part  with  the  more  rapid  or  intense  metabolism  stimulating  through 
conduction  regions  of  less  rapid  metabolism,  and  exciting  them  to  per- 
form their  peculiar  functions. 


PRINCIPLES  OF  VERTEBRATE  MORPHOLOGY       11 

The  Inhibiting  Influence  of  Dominant  Regions  Upon  the  De- 
velopment and  Degree  of  Differentiation  of  Subordinate  Regions. 

Among  the  invertebrates  that  have  the  same  three  fundamental 
axial  gradients  possessed  by  vertebrates,  experiments  have  shown  that 
during  development  a  dominant  region  exercises  an  inhibiting  in- 
fluence over  a  subordinate  region.  This  may  be  demonstrated  by 
experiments  in  regeneration.  If  a  planarian  worm  is  cut  transversely 
across  the  axis  of  polarity  at  almost  any  level  the  posterior  piece  will 
grow  a  new  head.  Evidently  then,  while  the  posterior  part  was  or- 
ganically connected  with  the  anterior  piece,  head  formation  was  in- 
hibited. As  soon  as  the  organic  connection  with  the  original  head  or 
dominant  part  has  been  severed,  the  inhibition  is  removed  and  a  head 
develops.  In  another  flatworm,  Microstomum,  the  dominance  of  the 
head  becomes  less  with  age  so  that  at  a  point  about  halfway  down  the 
axis  new  brain  and  eyes  form.  This  means  that  a  new  head  is  estab- 
lished, but  there  is  not  an  immediate  isolation  of  the  new  individual 
from  the  old.  As  long  as  the  new  individual  remains  a  part  of  the  old, 
it  remains  a  subordinate  individual,  not  only  functionally  in  following 
the  lead  of  the  original  head,  but  structurally  in  that  the  eyes  do  not 
fully  differentiate  and  the  brain  remains  small.  Each  of  the  two 
individuals  divides  again  into  two  and  in  each  case  the  posterior 
individual  is  subordinate  to  the  anterior. 

In  general  therefore  it  would  appear  that  the  presence  of  a  dominant 
part  inhibits  the  full  differentiation  of  subordinate  parts  in  proportion 
to  their  relative  metabolic  rates  or  intensities  and  the  proximity  of  the  two 
regions. 

Let  us  now  apply  these  principles  to  vertebrate  development.  In 
the  first  place  it  will  be  recalled  that  vertebrates  are  metameric 
animals  in  which  each  metamere,  at  least  back  of  the  primitive  head, 
is  serially  homologous  with  all  the  rest.  Each  metamere  has  therefore 
potentially  the  developmental  capacity  of  any  other.  Whatever  sys- 
tems or  structures  that  develop  in  any  one  metamere  should,  ex 
hypothese,  be  latent  in  all  of  the  others.  Any  failure,  therefore,  on  the 
part  of  a  given  metamere  to  realize  the  full  extent  of  its  differentia- 
tional  possibilities  must  be  attributed  to  some  sort  of  inhibition.  Let 
us  examine  the  main  regions  of  the  body  of  a  higher  vertebrate  with 
this  idea  in  mind. 

The  head  metameres  are  characterized  by  a  great  specialization  of 
brain  tissue  and  an  almost  complete  suppression  of  the  lower  functions. 


12  VERTEBRATE  ZOOLOGY 

In  the  neck  region  there  is  very  little  differentiation  of  subordinate 
elements  belonging  to  the  ventral  ranges  of  the  dorso-ventral  axis; 
muscles,  glands,  and  skeletal  elements  are  present,  but  no  excretory  or 
reproductive  organs  differentiate  in  that  region.  In  the  thoracic 
region,  where  the  dominance  of  the  anterior  end  of  the  dorsal  struc- 
tures is  less  intense,  the  organs  of  respiration,  circulation,  and  locomo- 
tion differentiate  fully;  but  there  are  no  digestive,  excretory,  or  re- 
productive specializations.  Still  further  back  in  the  upper  abdominal 
region  the  digestive  functions  reach  their  maximum  importance  and 
the  uro-genital  functions  begin  to  appear.  And,  finally,  in  the  lower 
abdominal  region,  the  excretory  and  genital  functions  develop  fully; 
but  the  genital  tissues  do  not  fully  differentiate  till  comparatively  late 
in  the  life  cycle. 

The  tail  of  the  vertebrate  appears  to  be  a  developmental  after- 
thought. It  is  the  last  part  to  develop  and  in  many  specialized  ver- 
tebrates scarcely  develops  at  all,  as  though  the  developmental  momen- 
tum slowed  down  to  such  an  extent  that  there  was  not  enough  force 
left  to  push  out  the  tail.  When  the  tail  does  form,  however,  it  is 
usually,  though  not  always,  the  most  primitive  part  of  the  body.  A 
tailless  condition  is  very  common  in  highly  specialized  races  with 
prolonged  developmental  period. 

GENERALIZED,    SPECIALIZED,    SENESCENT,  AND    RETARDED 
(P^DOGENETIC)   TYPES  OF   VERTEBRATES 

A  generalized  vertebrate  of  any  class  is  one  in  which  the  axial 
relations  are  well  in  balance;  the  primary  axis  distinctly  dominant 
over  the  secondary  and  tertiary  axes.  Such  animals  as  a  dog-shark, 
a  salamander,  a  lizard,  a  shrew,  are  typical  generalized  vertebrates. 
They  have  in  common  certain  characteristics  of  which  the  following 
are  the  most  important: — the  body  is  rather  elongated  and  cylindrical 
in  shape;  with  head,  trunk,  and  tail  in  normal  balance;  the  fore  and 
hind  limbs  of  approximately  equal  value;  and  no  pronounced  special- 
izations either  externally  or  internally.  Types  such  as  these  are 
usually  looked  upon  as  prototypic  of  the  groups  to  which  they  be- 
long and  therefore  as  affording  a  close  approximation  to  the  ancestral 
stock  from  which  the  group  in  question  has  been  derived. 

The  history  of  vertebrate  evolution  has  been  closely  associated 
with  a  series  of  radical  geographic  and  climatic  changes,  that  have 
had  the  effect  of  periodically  eliminating  large  numbers  of  specialized 


PRINCIPLES  OF  VERTEBRATE  MORPHOLOGY       13 

groups  which  have  become  adapted  to  a  peculiar  and  limited  en- 
vironment, and  of  permitting  only  a  few  plastic,  generalized  types, 
capable  of  adjusting  themselves  to  the  changed  world  conditions,  to 
persist.  These  generalized  forms  have  then  become  the  ancestral 
stock  from  which  the  specialized  types  of  the  new  period  have  arisen. 

There  has  usually  been  a  period  of  struggle  on  the  part  of  the  per- 
sisting generalized  species  to  gain  a  foothold;  then  they  have  mul- 
tiplied rapidly,  and  have  entered  upon  a  period  of  adaptive  radia- 
tion, which  has  resulted  in  the  development  of  terrestrial,  arboreal, 
aquatic,  fossorial,  and  volant  types.  Whether  an  animal  is  a  fish, 
amphibian,  reptile,  bird,  or  mammal,  it  meets  a  given  set  of  life  con- 
ditions in  much  the  same  manner.  The  fish-like  form,  for  example,  has 
been  adopted  not  only  by  fishes,  but  by  amphibians  (several  of  the 
persistently  aquatic  urodeles),  by  reptiles  (notably  by  the  extinct 
ichthyosaurs) ,  by  birds  (penguins  and  the  extinct  Odontolcse),  and 
by  mammals  (whales  and  dugongs).  These  all  have  certain  features 
in  common  (Fig.  4)  that  are  the  fundamental  adaptations  for  active 
life  in  the  water: — the  spindle-shaped  body,  fin-like  appendages, 
smooth,  water-shedding  exterior,  tail-fin  or  hind  limbs  modified  to 
act  as  a  propeller,  and  usually  dorsal  fins  (in  fish,  amphibia,  ichthy- 
osaurs, and  some  whales) .  Such  structures  that  serve  a  similar  func- 
tion in  adaptation  to  a  given  environmental  complex  are,  for  the 
most  part,  not  homologous,  but  merely  analogous.  They  are  techni- 
cally called  "homoplastic,"  which  implies  that  they  have  been 
molded  out  of  diverse  materials  into  like  forms. 

Every  successful  vertebrate  class  has  had  a  period  of  youth,  when 
the  members  were  all  comparatively  generalized;  a  period  of  maturity, 
characterized  by  adaptive  deployment  into  all  of  the  available  life 
zones;  and  a  period  of  old  age  or  senescence,  characterized  by  the 
development  of  bizarre,  overspecialized  types,  incapable  of  weathering 
a  world  crisis.  The  reptiles,  for  example,  arose  in  the  Palaeozoic  and 
were  at  first  generalized  lizard-like  forms.  Before  the  close  of  the 
Palaeozoic  there  had  arisen  many  specialized  and  a  few  precociously 
senescent  types,  most  of  which  became  extinct  during  the  troublous 
climatic  disturbances  that  ushered  in  the  Mesozoic.  Only  a  few  of  the 
more  generalized  reptilian  stocks  survived  the  crisis  and  adjusted 
themselves  to  the  new  conditions  of  the  Mesozoic,  the  Golden  Age 
of  the  reptiles.  This  age  saw  the  great  second  adaptive  radiation  of 
the  reptiles  and  the  production  of  many  overspecialized  or  senescent 


14  VERTEBRATE  ZOOLOGY 

types,  including  the  dinosaurs,  ichthyosaurs,  plesiosaurs,  and  ptero- 
saurs, all  of  which  became  extinct  before  the  close  of  the  Mesozoic. 
Only  a  few  of  the  more  generalized  stocks  lived  over  into  the  Cenozoic 
to  found  the  rather  conservative  modern  reptilian  classes. 

The  generalized  forms  are  like  the  conservative  germ-plasm  in 
heredity,  that  passes  on  from  generation  to  generation,  progressing 
slowly  and  steadily  along  definitely  directed  lines,  and  largely  un- 
influenced by  the  somatic  specializations  that  are  the  result  of  en- 
vironmental or  functional  adaptations.  Just  as  the  germ-plasm  is  a 
continuous,  unbroken  series  of  cell  generations  that  gives  off  tangen- 
tially  the  successive  somatic  generations  with  all  of  the  accompany- 
ing specializations,  so  is  the  generalized  stock  of  animal  forms  a  con- 
tinuous series  of  races  that  sprays  off  at  intervals  the  specialized 
types.  These  specialized  groups  go  their  ways  and  become  extinct, 
while  the  slowly  evolving  generalized  types  form  the  stock  from 
which  future  specialized  races  may  arise. 

Generalized  forms  are  never  equally  generalized  in  all  particulars. 
As  a  rule,  while  retaining  a  fundamentally  generalized  structure,  they 
become  superficially  specialized  in  one  or  more  particulars.  While 
the  ideally  generalized  type  is  only  approximated  in  any  class  of 
vertebrates,  some  one  or  more  forms  stand  out  from  their  fellows  as 
more  nearly  prototypic  than  the  rest.  Such  forms  are  of  great  phylo- 
genetic  interest  and  will  subsequently  claim  a  large  share  of  our  atten- 
tion. 

Not  infrequently  forms  that  appear  to  have  some  of  the  ear-marks 
of  prototypes  prove  on  examination  to  be  pretenders,  in  that  they  are 
either  secondarily  simplified  by  parasitic  or  sedentary  life,  or  are 
retarded  forms  that  have  failed  to  complete  the  life  cycle  normal  for 
the  group  to  which  they  belong.  Usually  the  degenerate  forms  may 
readily  be  recognized  by  the  study  of  their  life  history;  for  their  larval 
stages  show  more  advanced  conditions  of  various  structures  than  do 
the  definitive  forms.  The  retarded  forms,  on  the  other  hand,  simply 
cease  to  develop  somatically  in  a  larval  or  juvenile  stage,  while  the 
germinal  cycle  completes  itself.  As  a  consequence  these  juvenile 
forms  become  sexually  mature  and  reproduce  their  kind,  a  phenom- 
enon known  as  pcedogenesis  or  neoteny. 

Specialized  types  should  be  carefully  discriminated  from  senescent 
types,  though  few  specialized  forms  are  free  from  the  evidences  of 
senescence.  Specializations  are  to  be  looked  upon  as  adaptive  in 


PRINCIPLES  OF  VERTEBRATE  MORPHOLOGY       15 

character,  and  their  origin  as  a  sort  of  response  to  the  demands  of 
certain  environmental  conditions.  Senescent  characters,  although 
sometimes  apparently  adaptive,  are  frequently  valueless  or  distinctly 
disadvantageous  to  their  possessors,  and  have  often  been  responsible 
for  racial  extinction.  The  various  types  of  adaptive  complexes  and 
the  several  criteria  of  racial  senescence  must  receive  separate  consid- 
eration. 

SPECIALIZATIONS  AND  ADAPTATIONS 

It  has  been  pointed  out  that  a  primitive  group,  after  weathering  a 
radical  world  change  and  thus  surviving  its  more  highly  specialized 
relatives,  tends  to  begin  a  new  adaptive  deployment  and  to  occupy  all 
of  the  available  life  zones.  In  order  to  enter  specialized  life  zones  or 
live  upon  very  restricted  types  of  food,  adaptive  specializations  must 
occur  that  fit  various  offspring  of  the  primitive  stock  to  occupy  these 
restricted  life  conditions. 

In  general  it  may  be  said  that  the  active,  predaceous  life  is  primi- 
tive as  compared  with  the  more  passive  herbivorous  life  and  that, 
in  the  case  of  vertebrates  at  least,  the  generalized  types  are  for  the 
most  part  active  and  predaceous,  characterized  by  quick  action, 
keen  sensitivity,  sharp  grasping  teeth,  claws  as  opposed  to  hoofs, 
and  normal  balance  of  head,  trunk,  and  tail.  Bottom-feeding 
types  in  the  waters  and  browsing,  herbivorous  types  on  land  are 
characterized  by  slow  action  (though  many  herbivorous  types  have 
become  secondarily  cursorial),  cutting  and  grinding  teeth,  hoofs  in- 
stead of  claws,  and  lack  of  balance  in  bodily  proportions. 

Many  of  the  senile  races  are  characterized  by  the  possession  of 
armatures  of  various  sorts  that  are  unquestionably  of  great  defensive 
value  to  sluggish  and  otherwise  defenieless  creatures.  In  spite  of  the 
value  of  such  structures  to  their  possessors,  they  are  rather  to  be 
thought  of  as  the  products  of  racial  aging  than  as  responses  on  the 
part  of  the  organism  to  life  needs.  Only  in  a  very  limited  sense,  then, 
can  such  structures  be  classed  as  adaptations,  for  many  bony  and  scaly 
excrescences  of  senile  types  have  gone  far  beyond  the  bounds  of  use- 
fulness and  have  been  a  mere  burden  and  useless  incumbrance  to 
their  owners. 

In  the  body  of  this  volume  many  examples  of  adaptive  speciali- 
zations and  equally  numerous  illustrations  of  the  structures  resulting 
from  racial  senescence  will  be  cited  and  commented  upon.  In  this 


16  VERTEBRATE  ZOOLOGY 

place  we  shall  concern  ourselves  with  a  study  of  the  laws  of  adapta- 
tion, following  closely  the  analysis  given  by  Osborn. 

THE  LAWS  OF  ADAPTATION 

"The  form  evolution  of  the  back-boned  animals,  beginning 
with  the  pro-fishes  of  Cambrian  and  pre-Cambrian  time,  extends 
over  a  period  estimated  at  not  less  than  30,000,000  years.  The 
supremely  adaptable  vertebrate,  body  type  begins  to  dominate 
the  living  world,  overcoming  one  mechanical  difficulty  after  an- 
other, and  passes  through  the  habitat  zones  of  water,  land,  and 
air.  Adaptations  in  the  motions  necessary  for  the  capture,  stor- 
age, and  release  of  plant  and  animal  energy  continues  to  control 
the  form  of  the  body  and  its  appendages,  but  simultaneously  the 
organsim  through  mechanical  and  chemical  means  protects  it- 
self either  offensively  or  defensively  and  also  adapts  itself  to  re- 
produce and  protect  its  kind."  Osborn  finds  that  there  are  two 
great  laws  of  adaptation: 

1.  The  law  of  convergence  or  parallelism  of  form  in  locomotor,  offen- 
sive, and  defensive  adaptations.  There  is  a  widespread  tendency  for 
members  of  totally  unrelated  groups  to  develop  similar  body  form 
and  similar  external  characters  in  adaptation  to  similar  habitat  zones. 
Several  adaptive  types  are  readily  distinguished  and  are  herewith 
listed : 

Aquatic  or  swimming  animals.  Among  the  thoroughly  marine 
types  of  vertebrates,  whether  they  be  fish,  reptile  or  mammal,  there 
is  in  general  the  same  fish-like  body  form,  with  the  same  sorts  of 
locomotor  structures,  paired  and  median  fins.  Apart  from  their 
general  aquatic  adaptations  the  shark,  ichthyosaur  (reptile),  and 
dolphin  (mammal)  are  profoundly  different  (Fig.  4). 

"The  three  mechanical  problems  of  existence  in  the  water  hab- 
itat are:  first,  overcoming  the  buoyancy  of  water  either  by 
weighting  down  or  increasing  the  gravity  of  the  body  or  by  the 
development  of  special  gravitating  organs,  which  enable  animals 
to  rise  and  descend  in  this  medium;  second,  the  mechanical  prob- 
lem of  overcoming  the  resistance  of  water  in  rapid  motion,  which 
is  accomplished  by  means  of  warped  surfaces  and  well-designed 
entrant  and  re-entrant  angles  of  the  body  similar  to  the  '  stream- 
lines '  of  the  fastest  modern  yachts;  third,  the  problem  of  propul- 
sion of  the  body,  which  is  accomplished,  first,  by  sinuous  motion 


PRINCIPLES  OF  VERTEBRATE   MORPHOLOGY 


17 


of  the  entire  body,  terminating  in  powerful  propulsion  by  the  tail 
fin;  secondly,  by  supplementary  action  of  the  four  lateral  fins; 


FIG.  4. — Three  aquatic  types  of  vertebrate,  to  illustrate  convergent  adaptation 
of  three  wholly  unrelated  forms  for  marine  life.  All  three  show  the  fusiform  body, 
median  and  paired  fins,  though  the  skeletal  structures  are  radically  different. 
A,  shark  (Pisces);  B,  ichthyosaur  (Reptilia);  C,  porpoise  (Mammalia).  (After 
Osborn's  "  Origin  and  Evolution  of  Life  "  [Charles  Scribner's  Sons].) 

third,  by  the  horizontal  steering  of  the  body  by  the  median  sys- 
tem of  fins." 


18  VERTEBRATE  ZOOLOGY 

Flying  or  parachuting  animals,  belonging  to  all  of  the  vertebrate 
classes  except  the  cyclostomes,  exhibit  similar  parallelisms  which  con- 
sist of:  first,  planes  for  the  support  of  the  weight  in  the  air;  and  sec- 
ond, tapering  body-form  for  decreasing  the  resistance  of  the  body  in 
rapid  motion  through  the  air;  third,  a  rigid  framework  for  the  planes. 

Arboreal  or  climbing  (scansorial)  animals  of  all  vertebrate  classes 
have  one  prime  requisite — clinging  appendages.  Some  cling  by 
means  of  prehensile  fingers  or  tail,  others  by  the  use  of  adhesive  pads, 
others  by  air-suction  pads,  and  still  others  by  hook-like  claws. 

Running  (cursorial)  animals  are  very  much  alike  in  their  mechan- 
ical adaptations.  These  are  primarily  foot  modifications.  The  prime 
requisite  is  a  long  limb  with  plenty  of  spring  to  it.  This  requisite  is 
met  by  standing  high  on  the  toes  with  the  legs  well  under  the  body. 
Usually  there  is  a  progressive  loss  of  some  of  the  digits,  a  reduction 
that  reaches  its  climax  in  the  one-toed  horses. 

Digging,  burrowing  (fossorial)  animals  are  usually  long  and  narrow 
so  as  to  require  only  a  narrow-bore  burrow.  The  fore  limbs  are 
strong  and  have  a  heavy  shoulder  girdle  for  the  attachment  of  the 
heavy  digging  muscles.  Eyes  and  ears  are  usually  reduced  and  the 
tail  is  usually  poorly  developed. 

Desert-dwellers. — The  prime  requisites  for  these  animals  are: 
first,  coverings  of  some  sort  that  prevent  undue  loss  of  moisture, 
such  as  heavy  scales,  spines,  or  armor;  second,  protection  against  the 
extremes  of  temperature.  Many  of  them  meet  the  latter  require- 
ment by  burrowing  in  sand  or  in  the  soil  at  night.  It  is  also  very 
common  for  desert  creatures  to  be  venomous.  This  venom  may  be 
a  chemical  end-product  of  life  under  arid  conditions. 

Cave  and  deep  sea  animals. — These  animals  may  be  viewed 
largely  as  products  of  lowered  vitality.  It  can  scarcely  be  claimed  that 
their  characters  are  on  the  whole  adaptive.  One  character  very  com- 
monly found  in  abyssmal  creatures  is  phosphorescence.  Light-pro- 
ducing organs  of  all  sorts  are  developed  by  these  creatures  that 
appear  to  be  definitely  adapted  to  life  in  the  dark.  Possibly  phos- 
phorescence may  be  one  of  the  physiological  accompaniments  of  life 
under  abyssmal  conditions.  Deep-sea  vertebrates  (fishes)  are  also 
almost  invariably  forms  exhibiting  radical  distortions  of  the  general- 
ized fish  form.  Two  principal  types  are  common :  those  with  sup- 
pressed heads  and  those  with  exaggerated  heads.  This  condition  is 
discussed  elsewhere. 


PRINCIPLES  OF  VERTEBRATE  MORPHOLOGY       19 

Ant-eating  adaptations. — Ants  for  a  long  time  have  been  extremely 
numerous  and  have  been  an  important  factor  in  the  environment  of 
all  land  vertebrates  since  the  establishment  of  the  reptilian  orders. 
Wheeler  in  his  classic  treatise  on  ants  claims  that  many  vertebrate 
integumentary  structures,  such  as  hair,  feathers,  and  scales  are  pri- 
marily for  protection  against  ants.  Whether  this  position  is  justi- 
fied or  not,  it  is  certainly  true  that  ant-eaters  among  all  of  the  classes 
of  land  vertebrates  are  heavily  armored  or  densely  covered  with  in- 
tegumentary structures.  Besides  this  most  obvious  adaptation,  ant- 
eaters  have  long  slender  snout,  with  small  terminal  mouth,  long 
sticky  tongue,  teeth  reduced  or  absent,  strong  digging  feet,  and  in- 
ternal nostrils  far  forward  and  in  such  relation  to  the  glottis  that 
ants  could  not  possibly  crawl  into  the  wind-pipe. 

2.  The  law  of  divergence  of  form;  the  law  of  adaptive  radiation. 
This  law  is  the  antithesis  of  the  "Law  of  Convergence";  for,  in- 
stead of  a  similarity  of  adaptive  character  acquired  by  unrelated 
groups,  we  have  diversity  of  adaptive  characters  acquired  by  members 
of  a  single  related  stock,  which  tend  to  radiate  into  all  of  the  avail- 
able life  zones  and  to  develop  the  various  adaptive  complexes.  For 
example,  a  primitive  stock  of  cursorial  reptiles  splits  up  into  fossorial,. 
aquatic,  arboreal,  volant,  and  giant  herbivorous  and  carnivorous 
types.  A  type  once  arboreal  may  become  a  volant  type,  and  that 
seems  to  be  the  usual  sequence.  A  volant  type  does  not  become 
fossorial,  but  may  return  to  a  cursorial  habit,  as  for  example,  the 
running  birds.  An  aquatic  type  may  become  terrestrial  and  then 
secondarily  return  to  the  life  in  the  water,  carrying  back  with  it, 
however,  an  air-breathing  mechanism.  When  the  aquatic  verte- 
brates developed  adaptations  for  land  life  they  lost  their  gills,  their 
lateral-line  organs,  etc.  When  once  a  character  is  lost  it  cannot  be 
regained,  so  the  aquatic  reptiles  and  mammals  must  be  dependent 
on  lungs,  although  they  would  be  much  better  off  with  gills.  This 
illustrates  the  irreversibility  of  evolutionary  changes,  and  especially 
of  adaptive  specializations.  Osborn,  in  his  book  on  "The  Origin 
and  Evolution  of  Life,"  takes  the  position  that  the  irreversibility 
of  evolution  is  due  to  the  progressive  chemical  evolution  of 
the  "heredity  chromatin."  Its  changes,  he  believes,  are  orderly  and 
progress  step  by  step  toward  more  and  more  narrow  specializa- 
tion. Once  the  chromatin  has  acquired  factors  for  specialized 
characters,  it  cannot  reverse  and  return  to  the  generalized  condi- 


20  VERTEBRATE  ZOOLOGY 

tion.  All  it  can  do  is  to  lose  certain  old  characters  and  develop  new 
ones  of  a  different  sort.  A  too  narrowly  specialized  creature  is  at 
the  mercy  of  a  changing  geologic  age.  Rather  sudden  climatic  and 
geographic  changes  have  been  the  rule  in  geologic  history,  and  when- 
ever such  changes  have  occurred  there  has  been  a  rapid  extinction 
of  the  most  highly  specialized  types,  races  that  are  no  longer  plastic 
enough  to  adjust  themselves  by  adaptations  to  the  new  conditions. 
They  are  in  a  "cul-de-sac  of  structure,"  says  Osborn,  "from  which 
there  is  no  possible  emergence  by  adaptation  to  a  different  physical 
environment  or  habitat.  It  is  these  two  principles  of  too  close  adjust- 
ment to  a  single  environment  and  of  the  non-survival  of  characters 
once  lost  by  the  chromatin  which  underlie  the  law  that  the  highly 
specialized  and  most  perfectly  adapted  types  become  extinct,  while 
primitive,  conservative,  and  relatively  unspecialized  types  invari- 
ably become  the  centres  of  new  adaptive  radiation." 

RACIAL  SENESCENCE  AND  DEGENERATION 

Just  as  the  individual  grows  old  and  suffers  a  retardation  of  all  vital 
activities,  so  races  age  and  show  similar  evidences  of  lowered  vitality 
and  diminished  activity.  In  young  individuals  and  in  young  races  the 
rate  of  metabolism  is  high  and  the  expressions  of  a  high  rate  of  metab- 
olism (a  more  intense  vitality)  are  seen  in  their  comparatively  active 
life,  predaceous  habits,  structures  on  the  whole  generalized,  moderate 
size,  and  lack  of  heavy  excrescences.  When  the  individual  or  the  race 
is  young,  the  products  of  its  metabolism  are  used  up  largely  in  motor 
activities  of  various  sorts  and  there  is  little  deposition  of  inert  mate- 
rials such  as  armor,  spines,  heavy  bones,  fats,  or  massive  flesh. 

A  senescent  race,  on  the  other  hand,  is  characterized  by  sluggish 
behavior,  by  herbivorous  habits  or  feeding  habits  involving  little 
exertion,  by  structures  on  the  whole  specialized  or  degenerate,  often 
by  giant  size  or  bulky  build,  and  by  accumulations  of  inert  materials 
such  as  armor,  spines,  heavy  bones  or  flesh.  These  and  other  char- 
acters are  now  very  generally  recognized  as  criteria  of  racial  senescence. 

STRUCTURAL  CRITERIA  OF  RACIAL  SENESCENCE 

Large  Size. — It  has  been  found  that  the  highly  specialized  races  of 
the  past  have  usually  grown  to  giant  size  as  compared  with  their  less 
specialized  relatives.  Good  examples  of  this  phenomenon  are  seen 
in  the  great  dinosaurs,  and  in  the  monster  mammals  of  the  past  and  of 


PRINCIPLES  OF  VERTEBRATE  MORPHOLOGY       21 

the  present,  such  as  the  proboscidians  and  the  whales.  In  these  forms 
growth  has  run  riot,  probably  because  of  the  lack  of  growth-inhibiting 
factors. 

Spinescence. — In  both  vertebrates  and  invertebrates  the  develop- 
ment of  spines,  horns  and  other  chitinous  or  bony  excrescences  are 
generally  believed  to  be  evidences  of  racial  old  age.  As  the  general 
metabolism  slows  down  there  appears  to  be  a  tendency  for  hard  or 
dead  substances  to  be  deposited  in  regions  of  lowest  metabolic  rate, 
just  as  debris  collects  in  an  eddy.  Good  examples  of  this  phenomenon 
are  found  in  Stegosaurus,  in  Edaphosaurus,  in  the  horned  toads,  in 
many  teleost  fishes,  in  porcupines,  and  in  the  deer  family.  In  the  last- 
named  group  the  extinct  Irish  elk  stands  out  as  a  classic  example  of 
a  type  that  went  a  step  too  far  in  the  elaboration  of  excrescences,  and 
became  extinct. 

Degeneracy. — One  of  the  systems  most  commonly  found  in  a  de- 
generating condition  is  the  dentition.  Many  of  the  most  highly 
specialized  groups  have  undergone  a  partial  or  total  loss  of  teeth. 
This  is  true  of  the  edentates  (archaic  types  of  placental  mamals),  of 
the  turtles  (among  the  oldest  and  most  specialized  among  reptilian 
orders),  of  the  sturgeon  (a  degenerate  end  product  of  a  long  line  of 
chondrostean  fishes),  and  of  modern  birds  (possibly  the  most  highly 
specialized  vertebrates).  Loss  of  teeth  is  equally  common  among  the 
extinct  groups  of  the  past,  such  as  the  beaked  dinosaurs,  and  the 
great  sauropods.  Degeneration  of  the  tail  is  almost  as  common 
among  highly  specialized  and  senescent  races  as  is  the  loss  of  teeth. 
Examples  of  taillessness  are  too  numerous  to  list.  Loss  of  limbs  is 
also  very  common  in  senescent  types. 

The  Eel-like  Form. — Lull  also  recognizes  as  a  criterion  of  racial 
old  age  great  bodily  elongation,  usually  accompanied  by  partial  or 
total  loss  of  limbs.  Gregory  lists  forty-four  distinct  types  of  eel-like 
creatures  among  the  vertebrates,  including: — three  groups  of  cyclo- 
stomes,  one  of  sharks,  a  lung  fish,  three  teleostome  groups,  three 
amphibian  groups,  five  reptilian  groups,  and  one  group  of  mammals. 
In  all  cases  the  criteria  are  about  the  same,  though  the  degree  of  elon- 
gation and  reduction  of  limbs  varies  greatly. 

Elaborate  Coloration. — Most  primitive  or  generalized  groups  of 
animals  have  dull  colors  and  indistinct  patterns;  but  one  of  the  most 
striking  features  of  highly  specialized  climax  groups  of  vertebrates, 
such  as  the  teleost  fishes  and  the  birds,  is  their  remarkably  high  color- 


22  VERTEBRATE  ZOOLOGY 

ation.  The  more  brilliant  pigments  may  be  looked  upon  as  end  prod- 
ucts of  certain  chemical  processes  in  metabolism,  and  therefore 
appropriate  in  groups  that  represent  end  products  of  long  lines  of 
specialization. 

Possible  Internal  Causes  of  Some  Phases  of  Senescence. — 
The  effects  of  atrophy  or  hypertrophy  of  such  endocrine  glands  as 
the  thyroid,  pituitary  body  (hypophysis),  ovaries  or  testes,  upon 
growth  and  differentiation,  are  well  known.  Disturbances  in  the  nor- 
mal functionality  of  these  various  glands  underlie  giantism,  dwarfism, 
degeneracy,  failure  to  complete  the  differentiation  of  the  secondary 
sexual  characters,  etc.  It  is  not  unlikely  that  racial  senescence,  like 
individual  senescence,  may  be  the  result  of  a  progressive  deteriora- 
tion of  the  germinal  determiners  of  these  important  balance-wheels  of 
organic  growth.  It  is  also  not  unlikely  that  most  of  the  changes  in 
bodily  proportion,  which  constitute  a  vast  preponderance  of  specific 
and  racial  differences,  are  largely  due  to  relative  racial  atrophy  or 
hypertrophy  of  these  growth-regulating  glands.  The  giant  races  may 
be  those  in  which  thyroid  abnormality  has  unloosed  the  restraints  to 
growth,  and  size  has  reached  the  limit  of  mechanical  possibility.  Sim- 
ilarly racial  thyroid  or  hypophysis  atrophy  may  account  for  degen- 
erate types.  An  excellent  example  of  the  effectiveness  of  thyroid  in 
controlling  differentiation  is  seen  in  the  case  of  frog  development. 
The  bull-frog  larva  in  northern  waters  takes  several  years  to  reach  the 
stage  of  metamorphosis;  but  if  fed  upon  thyroid  there  is  a  precocious 
metamorphosis  of  the  first-year  larva  while  it  is  still  only  about  one- 
fourth  the  size  normally  reached  in  nature.  Thus  giantism  in  frog 
larvae  is  the  result,  presumably,  of  deficient  or  belated  thyroid  secre- 
tion. The  converse  of  this  experiment  has  been  made  by  extirpation 
of  the  hypophysis  in  tadpoles.  Allen  has  found  that  these  operated 
larvae  do  not  develop  a  thyroid  and  are  unable  to  metamorphose. 
The  bearing  of  these  experiments  on  the  phenomenon  of  neoteny  or 
psedogenesis  is  obvious. 

VERTEBRATE  PHYLOGENY 

Phylogeny  may  be  defined  as  the  science  of  ancestries  or  genealogies, 
and,  as  such,  is  one  of  the  chief  concerns  of  the  present  book.  In 
attempting  to  discover  the  origin,  ancestry,  and  relationships  of  a 
given  group  of  animals,  we  employ  mainly  three  sets  of  criteria,  which 
for  the  sake  of  brevity  we  may  term:  homologies,  ontogenies,  and 


PRINCIPLES  OF  VERTEBRATE  MORPHOLOGY       23 

fossils.  These  three  phenomena  form  the  chief  subject-matter  of 
the  sciences  of  comparative  anatomy,  embryology,  and  palaeontology. 

HomolcTgies  have  already  been  dealt  with  incidentally  in  the  fore- 
going discussion  of  adaptations;  for  the  second  law  of  adaptation, 
"the  law  of  divergence  of  form,"  implies  homology,  since  the  variously 
modified  adaptive  structures  are  the  products  of  a  single  generalized 
ancestral  prototype,  and  are  strictly  comparable  in  fundamental 
structure  and  mode  of  origin.  Thus  the  wing  of  a  bird,  the  fore  leg 
of  a  horse,  and  the  flipper  of  a  whale  are  homologous  in  spite  of  their 
profound  departures,  both  structurally  and  functionally,  from  the 
generalized  vertebrate  fore  limb.  Similarly,  the  rudimentary  hind 
limbs  of  a  whale  and  the  splint  bones  of  a  horse  are  recognized  as 
homologues  of  the  fully  functional  structures  of  allied  groups. 

We  must  be  constantly  on  our  guard  against  the  all  too  common 
error  of  mistaking  for  homologies  " convergences  of  form"  such  as 
are  dealt  with  in  Osborn's  first  law  of  adaptation.  Thus,  while  the 
shark,  ichthyosaur,  and  porpoise  appear  to  possess  homologous 
adaptations  in  their  general  form  and  locomotor  organs,  such  struc- 
tures as  the  caudal  and  dorsal  fins  in  these  three  types  are  more  truly 
analogous  than  homologous,  for  they  do  not  represent  the  same  em- 
bryonic rudiments,  nor  are  they  made  of  the  same  structural  materials. 
Before  we  can  be  certain  about  homologies  we  must  put  them  to  the 
test  by  means  of  a  study  of  their  embryonic  development. 

EMBRYOLOGICAL  ASPECTS  OF  PHYLOGENY 
THE  RECAPITULATION  THEORY 

This  theory  has  often  been  called  Haeckel's  Biogenetic  Law,  and 
may  be  paraphrased  as  follows:  The  life  history  of  the  individual 
gives  a  brief  condensed  resume  of  the  evolutionary  history  of  its  an- 
cestors. Some  one  has  said  that  "the  individual  climbs  its  own 
ancestral  tree."  In  a  somewhat  restricted  and  modified  sense  this 
theory  is  still  accepted  by  most  embryologists,  though  there  are  some 
who  have  cast  it  aside  as  worthless.  Embryology  has  been  compared 
to  "an  ancient  manuscript  with  many  sheets  lost,  other  displaced, 
and  with  spurious  passages  interpolated  by  a  later  hand."  The  fact 
that  the  history  is  tremendously  abbreviated  precludes  the  recapitula- 
tion of  every  ancestral  stage.  The  necessity  for  the  embryo  to  prepare 
quickly  to  meet  a  difficult  environment  and  to  obtain  its  own  liveli- 


24  VERTEBRATE  ZOOLOGY 

hood  causes  the  pushing  back  into  the  earlier  stages  of  some  of  the 
characters  that  may  have  had  a  later  phylogenetic  origin,  and  also 
calls  forth  the  development  of  new  characters  that  are  merely  adapta- 
tions for  larval  life.  Characters  that  are  considered  truly  ancestral 
are  known  as  palingenetic,  and  those  that  are  mere  interpolations 
for  purposes  of  larval  adaptation  are  known  as  ccenogenetic.  A  good 
example  of  a  palingenetic  life  history  is  that  of  the  common  frog, 
but  even  here  there  are  certain  larval  characters  that  are  purely 
csenogenetic.  In  some  of  the  non-aquatic  frogs  in  which  the  eggs  are 
not  laid  in  the  water  the  typical  cycle  is  greatly  foreshortened  through 
the  omission  of  the  larval  stages;  the  young  hatches  out  as  a  little 
frog  with  only  a  mere  stump  of  a  tail  left.  This  type  of  foreshortening 
the  life  history  is  known  as  tachygenesis. 

It  will  appear  from  what  has  just  been  said  that  some  ontogenies 
may  be  fairly  reliable  recapitulations  of  phylogenies,  but  that  others 
are  entirely  unreliable  as  guides  in  phylogenetic  matters.  The  ques- 
tion may  well  be  asked  as  to  whether  we  are  in  a  position  to  decide 
in  any  given  case  whether  the  embryonic  history  is  truly  palingenetic 
or  not.  In  answer  to  this  we  may  say  that  the  evidence  of  embry- 
ology is  valuable  only  when  it  is  supported  by  a  study  of  comparative 
anatomy  and  palaeontology. 

One  point  about  the  recapitulation  theory  that  is  seldom  clearly 
apprehended  has  been  clearly  stated  by  Lillie: 

"If  phylogeny  is  to  be  understood  to  be  the  succession  of  adult 
forms  in  the  line  of  evolution,  it  cannot  be  said  in  any  real  sense 
that  ontogeny  is  a  brief  recapitulation  of  phylogeny,  for  the  em- 
bryo of  a  higher  form  is  never  like  the  adult  of  a  lower  form, 
though  the  anatomy  of  embryonic  organs  of  higher  species  re- 
sembles in  many  particulars  the  anatomy  of  homologous  organs 
of  the  adult  of  the  lower  species.  However,  if  we  conceive  that 
the  whole  life  history  is  necessary  for  the  definition  of  a  species, 
we  obtain  a  different  basis  for  the  recapitulation  theory.  The 
comparable  units  are  then  entire  ontogenies,  and  these  resemble 
one  another  in  proportion  to  the  nearness  of  relationship,  just  as 
the  definitive  structures  do.  Thus  in  nearly  related  species  the 
ontogenies  are  very  similar;  in  more  distantly  related  species 
there  is  less  resemblance,  and  in  species  from  different  classes 
the  ontogenies  are  widely  divergent  in  many  respects." 
It  must  not  be  forgotten,  however,  that  the  ontogenies  of  closely 


PRINCIPLES  OF  VERTEBRATE  MORPHOLOGY       25 

related  species  may  be  made  to  differ  markedly  from  one  another  by 
caenogenetic  adaptive  changes,  as  in  the  case  of  some  of  the  frogs  that 
have  adopted  the  habit  of  laying  the  eggs  out  of  water.  In  these 
cases  the  ontogenies  are  much  more  widely  divergent  than  one  would 
expect  in  forms  so  closely  related.  So  the  statement  of  Lillie  that 
"ontogenies  resemble  one  another  in  proportion  to  the  nearness  of 
relationship,"  might  be  emended  by  adding  the  clause: — unless  the 
ontogenies  have  been  secondarily  disturbed  by  adaptive  ccenogenetic  modi- 
fications. In  all  of  our  attempts  to  make  use  of  ontogenies  as  evi- 
dences of  phylogenetic  relationships  we  must  be  on  our  guard  against 
the  various  pitfalls  that  have  been  discussed,  and  must  not  fail  to 
recognize  that  in  phylogenetic  research  palaeontology  is  a  more  re- 
liable guide  than  is  embryology. 

THE  GEOLOGIC  ASPECTS  OF  VERTEBRATE  PHYLOGENY 

In  order  to  appreciate  the  historical  side  of  vertebrate  evolution 
it  is  necessary  to  know  something  about  the  geologic  succession  of  the 
various  classes.  It  is  estimated  that  at  least  25,000,000  years  have 
elapsed  (some  authors  estimate  ten  times  this)  since  the  deposition 
of  the  strata  in  which  we  find  the  earliest  vertebrate  fossils.  Very 
likely  at  least  5,000,000  years  of  evolutionary  progress  had  been 
made  since  the  first  chordate  ancestors  of  these  early  armored  verte- 
brates had  made  their  appearance.  The  accompanying  chart  (Fig.  5) 
shows  in  compact  form  the  great  eras  of  geologic  time  and  the  number 
of  years  that  each  is  believed  to  have  occupied.  Another  excellent 
chart  by  Osborn  (Fig.  6)  indicates  the  characteristic  vertebrates  of  the 
various  great  geologic  ages  and  the  successive  geologic  appearance, 
adaptive  radiations  and  diminutions  of  the  five  vertebrate  classes. 
It  will  be  noted  that  all  of  the  classes  arise  in  the  Palaeozoic,  that 
each  higher  class  arises,  not  late  in  the  developmental  history  of  the 
ancestral  class,  but  very  near  its  base,  when  the  earlier  race  was  young. 
Thus  the  Amphibia  are  shown  coming  off  from  fishes  of  the  De- 
vonian; the  reptiles  from  the  Lower  Carboniferous  Amphibia  before 
the  Amphibia  themselves  had  made  any  progressive  adaptive  radia- 
tion; while  both  birds  and  mammals  came  off  near  the  base  of  the 
reptilian  trunk,  the  birds  probably  somewhat  later  than  mammals. 
In  addition,  it  will  be  noted  that  the  first  vertebrate  remains  occur 
far  down  in  the  Palaeozoic,  in  Ordovician  times,  which  were  char- 
acterized primarily  by  invertebrate  forms.  The  student  will  find  it 


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FIG.  5. — Total  Geologic  Time  Scale,  estimated  at  sixty  million  years. 
Osborn's  "Origin  and  Evolution  of  Life  "  [Charles  Scribner's  Sons].) 

26 


(After 


PRINCIPLES  OF  VERTEBRATE   MORPHOLOGY 


27 


of  great  advantage  to  memorize  the  geologic  epochs  and  ages  in  their 
order,  for  it  will  be  necessary  in  dealing  with  the  extinct  representa- 
tives of  the  vertebrate  classes  to  assign  them  without  explanation 
to  their  appropriate  geologic  periods. 

The  study  of  geology  teaches  us  that  the  earth's  outer  zones  have 
undergone  within  the  period  of  vertebrate  history  numerous  pro- 
found changes  which  in  general  we  may  term  climatic  changes. 


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FIG.  6. — Chronological  chart  of  vertebrate  succession.  Successive  geologic 
appearance  and  epochs  of  maximum  radiation  (expansion)  and  diminution  (con- 
traction) of  the  five  classes  of  vertebrates,  namely,  fishes,  amphibians,  reptiles, 
birds,  and  mammals.  (After  Osborn's  "  Origin  and  Evolution  of  Life  "  [Charles 
Scribner's  Sons].) 

There  have  been  periods  of  continental  subsidence,  accompanied 
by  ocean  floor  elevations,  during  which  the  great  continental  plains 
have  been  covered  by  comparatively  shallow  seas.  The  marine 
faunas  of  the  seas  have  migrated  into  these  shallows,  and  representa- 
tives of  them  have  been  buried  in  sediment.  When  the  reverse 
change  has  occurred  and  the  continental  plains  have  been  again  ele- 
vated, the  sedimentation  of  the  shallow-sea  period  forms  a  great 
rocky  stratum  laden  with  marine  fossils.  Between  periods  of  sub- 
sidence millions  of  years  elapsed,  and  therefore  a  break  in  the  conti- 
nuity of  the  entombed  fossils  is  to  be  expected.  Discontinuity  of 
fossil-bearing  strata  is  the  rule.  If  it  were  not  for  this  periodicity 


28  VERTEBRATE  ZOOLOGY 

of  subsidence  and  elevation  there  would  be  no  boundaries  between 
consecutive  geologic  strata. 

There  have  also  been  rhythms  of  alternating  aridity  and  humidity 
which  have  been  associated  with  marked  evolutionary  changes  in  land 
life.  The  principal  periods  of  mountain  uplift  are  shown  in  Fig.  7. 
One  of  the  most  significant  of  such  periods  was  the  great  Permo- 
Triassic  arid  period  which  saw  the  earliest  true  land  animals,  the 
reptiles,  make  their  first  adaptive  radiation  and  also  gave  rise  to  the 
first  mammalian  and  perhaps  avian  stocks. 

The  elevations  of  mountain  ranges  have  from  time  to  time  vastly 
altered  the  land  habitats,  especially  those  of  the  central  continental 
plains,  which  were  rendered  arid  by  being  cut  off  from  the  moisture 
of  the  sea.  Finally,  periodic  eras  of  cold  (glacial  epochs)  have  alter- 
nated with  tropical  or  semi-tropical  eras,  and  these  have  naturally 
exercised  a  profound  effect  upon  the  character  of  the  faunas  and 
floras.  In  general  the  coal  measures  serve  as  a  marker  for  the  trop- 
ical periods,  among  which  the  Upper  Carboniferous  and  Upper  Cre- 
taceous were  the  most  striking.  After  the  warm,  moist  climate  of  the 
Upper  Carboniferous  there  followed  rather  suddenly  the  Permian 
glacial  period  accompanied  by  continental  elevation  and  increased 
aridity.  There  was  no  such  sudden  change  at  the  end  of  the  Creta- 
ceous; but  during  that  period  there  were  extensive  continental  ele- 
vation, considerable  increase  in  aridity,  and  a  period  of  cold,  not 
severe  enough  however  to  be  termed  glacial. 

A  consideration  of  palseogeography  and  a  knowledge  of  the  cli- 
matic changes  during  the  vast  period  of  vertebrate  evolution  is  ab- 
solutely essential  for  an  intelligent  understanding  of  the  organic 
evolutionary  processes. 

It  seems  quite  clear  that  the  great  geologic  changes  have  been 
accompanied  by  equally  marked  changes  in  the  floras  and  faunas  of 
the  earth.  In  general  the  changes  in  the  organic  world  have  been 
of  such  a  kind  as  to  bring  about  adjustments  of  the  animals  and 
plants  to  the  changed  conditions.  What  the  exact  mechanics  em- 
ployed by  the  organism  in  making  these  responses  has  been,  has  never 
been  definitely  determined.  The  mechanics  of  adaptation  remains 
one  of  our  unsolved  problems.  One  theory,  that  of  Lamarck,  is  that 
the  organism  responds  directly  to  the  changed  environment  and  that 
the  germ-plasm  reflects  the  somatic  response,  so  that  the  change  be- 
comes a  permanent  racial  asset.  Another  theory  is  that  of  paral' 


PERIODS 


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PALISADE 

APPALACHIAN 


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FIG.  7. — Main  divisions  of  geologic  time.  The  notches  at  the  sides  of  the  scale  rep- 
resent chiefly  the  periods  of  mountain  uplift  in  the  northern  hemisphere  of  the  Old 
World  (left)  and  the  New  World  (right).  (From  Osborn's  "  The  Age  of  Mammals.") 

29 


30  VERTEBRATE  ZOOLOGY 

induction,  which  means  that  both  soma  and  germ  are  affected  simul- 
taneously and  in  the  same  way.  Perhaps  it  would  be  best  to  think 
of  the  organism  as  a  whole  and  view  the  response  as  a  general  organ- 
ismal  change  that  becomes  fixed  only  after  oft-repeated  exposure 
generation  after  generation.  In  some  cases  it  may  be  said  that  ex- 
ternal factors  simply  accelerate  or  retard  processes  that  were  already 
under  way  in  the  germ-plasm,  so  that  the  response  appears  to  be 
something  new  in  kind  when  it  is  only  the  result  of  a  sudden  accelera- 
tion of  a  character  evolution  already  under  way.  Whatever  be  the 
underlying  mechanism  involved  in  adaptive  changes,  there  is  no  hope 
of  explaining  adaptations  on  the  Darwinian  basis,  through  the  selec- 
tion of  the  best  out  of  a  vast  array  of  purely  fortuitous  variations; 
for  if  the  historical  study  of  vertebrate  evolution  reveals  one  thing 
more  clearly  than  any  other,  it  is  that  evolutionary  changes  are  orderly, 
progressive,  and  determinate  in  character,  and  that  in  many  re- 
spects these  orderly  processes  of  evolution  are  independent  of  each 
other  and  of  environmental  changes. 


CHAPTER  II 
THE  PHYLUM  CHORDATA 

The  characteristics  of  the  vertebrates  have  been  sufficiently  stated 
in  the  previous  chapter.  A  vertebrate  proper  may  be  readily  recog- 
nized by  applying  to  it  the  definition  on  page  1,  but  there  is  a 
considerable  assemblage  of  creatures,  which,  though  falling  short  of 
being  vertebrates  in  some  respects,  are  obviously  vertebrate-like  in 
others,  and  are  therefore  classed  with  the  Vertebrata  or  Craniata  in 
the  phylum  Chordata. 

These  creatures  range  all  the  way  from  worm-like,  burrowing  forms 
such  as  Balanoglossus  and  the  sessile,  tubicolous  Cephalodiscus, 
through  the  degenerate,  sedentary,  flask-like  tunicates,  to  the  pro- 
fish  Amphioxus.  When  we  include  the  vertebrates,  the  range  of 
structural  diversity  and  degree  of  specialization,  with  Rhabdopleura 
at  one  extreme  and  Man  or  the  whales  at  the  other,  is  so  great 
that  one  doubts  the  advisability  of  making  a  single  phylum  so  all- 
inclusive. 

If,  however,  we  adopt  as  the  specifications  of^aj^ordatp  t.hft  poa^ 
session  of  a  notochord,  pharyngeal  clefts,  and  a  meduftp.ry  plat,?^  we 
arbitrarily  throw  together  all  animals  that  possess  these  characters. 
It  therefore  becomes  essential  that  we  have  exact  criteria  for 
recognizing  these  characters  in  whatever  guise  they  may  be  pre- 
sented. 

Notochord. — The  notochord  is  recognized  by  its  position,  by  its 
histological  structure,  by  its  function,  and  by  its  embryonic  deriva- 
tion. Typically  it  is  a  stiff  hyaline  rod  (Fig.  8)  covered  with  a  con- 
nective tissue  sheath,  lying  ventral  to  the  neural  tube  and  dorsal  to 
the  alimentary  tract.  It  is  composed  of  vacuolated  cells  that  histo- 
logically  resemble  pith.  It  is  derived  from  a  median  dorsal  strip  of 
tissue  cut  off  from  the  prhnTEiVe  endo'cterm  or  archenteron. 

Pharyngeal  Clefts  (Gill-slits). — The  term  "pharyngeal  clefts"  is 
preferred  to  "gill-slits"  because  it  is  more  accurately  descriptive  and 
has  no  dubious  functional  implications.  The  pharynx  1  <$*&•  ^ 
simpK  °n  opening  through  the  body  wall  in  the  pharyngeal  region 
^C 


32  VERTEBRATE  ZOOLOGY 

that  allows  water  to  pass  out  of  the  pharynx  to  the  exterior.  The 
opening  is  made  embryonically  by  an  out-pouching  of  the"p!ia^TLX 
and  an  in-pouching  of  the  ectoderm,  which  meet  and  break  through. 
There  is  therefore  always  an  ectodermal  and  an  endodermal  part  of 
a  pharyngeal  cleft.  When  gills  are  formed  they  are  derived  sometimes 
from  the  ectodermal  and  sometimes  from  the  endodermal  part  of  the 
cleft.  In  terrestrial  vertebrates  no  gills,  except  minute  transitory 
rudiments  of  gill  filaments,  ever  develop,  though  the  pharyngeal 
clefts  may  appear  and  either  break  through  or  remain  closed.  The 
number  of  pharyngeal  clefts  ranges  from  upwards  ol  fifty  in  Amphi- 
oxus  to  one  pair  in  Cephalodiscus  and  none  in  Rhabdopleura.  Usually 
a  framework  of  cartilage  or  bones  constitutes  the  branchial  skeleton 
and  serves  to  support  the  clefts,  their  accompanying  branchial  tissues, 
and  their  blood  supply. 

Medullary  Plate  (Neural  Tube).— The  term  "medullary  plate"  is 
perhaps  somewhat  more  widely  applicable  than  "neural  tube"  be- 
cause in  some  of  the  more  primitive  chordates  the  plate  never  reaches 
the  tubular  condition.  The ^medullary  plale4s  a  dorsaLarea  of  ecto- 
dgrrn  that  typically  becomes  first  foM^d^jn_^ngitudinajlv^Jii1k)  a 
neural  groove  and  then  converted  into  a  tube  with^  a  central  canaj_or 
neurocotir. — !CT^uf  Lliu  VefteBrattes  piTtpeT^rt^soine  ^period  haveThe 
central  nervous  system  external  (in  the  medullary  plate  condition), 
but  some  of  the  more  primitive  chordates  have  merely  a  diffuse  plexus 
of  nerve  cells  in  the  skin  with  a  tendency  for  them  to  concentrate 
along  the  dorsal  side.  Such  individuals  can  hardly  be  credited  with  a 
medullary  plate,  much  less  a  neural  tube.  Some  of  their  immediate 
relatives,  however,  have  the  beginnings  at  least  of  an  infolding 
that  makes  it  probable  that  the  structure  concerned  is  a  medullary 
plate. 

In  dealing  with  the  various  groups  that  lay  claim  to  membership, 
along  with  the  vertebrates,  in  the  chordate  fraternity,  it  will  be 
necessary  to  test  their  claims  by  a  rigid  examination  of  their  creden- 
tials: notochord,  pharyngeal  clefts,  and  medullary  plate. 

The  heterogeneous  collection  of  types  that  has  been  assembled  by 
comparative  anatomists  and  called  chordates  is  customarily  sub- 
divided into  four  sub-phyla,  which,  beginning  with  the  group  that 
shows  most  certain  affinities  to  the  vertebrates  and  followed  by  those 
forms  that  haye  a  less  valid  claim  to  vertebrate  relationship,  ^re  as 
follows : 


/THE  PHYLUM   CHOftDATA  33 

Sub-Phylum  Tj~~Cephalochordata  (Adelochorda) . 

•This  includes  ,but  a  single  family  of  fish-like  creatures,  of 
which  there  are  about  twelve  speciesv     The  type  form  is 
1    Amphioxus  (more  correctly  known  as  Branchiostoma) . 

Sub-PEylum  II.    Urochordata. 

Order  1.  Larvacea  (Appendicularia),  free-swimming  forms 
with  permanent  tail. 

Order  2.  Ascidiacea  (Tunicates  or  Sea-Squirts),  fixed  forms 
without  tail  in  the  adult. 

Order  3.  Thaliacea  (Salpians),  free  swimming  forms  with- 
out tail  in  the  adult. 


Hemichordata. 
Order  1.     Enteropneusta,  including  worm-like  forms  such 

as  Balanoglossus.    - 
Order  2.     Pterobranchiata,  sessile,  tube-dwelling  forms — 

Cephalodiscus  and  Rhabdopleura. 
Order  3.    Phoronidia,  tubicolous  forms — Phoronis. 

Sub-Phylum  IV.    Vertebrata  (Craniata). 

Class  1.     Cyclostomata '  (round  mouth  eels): 

Class  2.     Pisces  (true  fishes  with  jaws). 

Class  3.     Amphibia  (vertebrates  with  aquatic  larvae,  bu 

usually  air-breathing  in  the  adult  condition). 
Class  4.     Reptilia  (cold-blooded,  air-breathing  vertebrates} 
Class  5.     Aves  (birds,  feathered  vertebrates). 

Class  6.     Mammalia  (beasts  or  quadrupeds) . 

I 

SUB-PHYLUM  I.    CEPHALOCHORDATA 

JThe  Cephalochordata  are  considered  first  because  their  claims  to 
vertebrate  relationship  lire  stronger  than  those  of  the  other  pro  ver- 
tebrate sub-phyla.  They  are  rather  small,  marine,  fish-like  animals, 
usually  called  "lancelets"  on  account  of  their  sharply  pointed  ends. 
Amphioxus  was  first  described  by  Pallas  in  1778.  On  account  of  its 
resemblance  to  a  slug  it  was  given  the  name  of  Limax  lanceolatus,  the 
implication  being  that  it  was  a  mollusk.  In  1804  Costa,  an  Italian 
naturalist,  redescribed  it  as  a  fish,  allied  to  the  lampreys  and  hag- 
fishes,  and,  because  he  erroneously  diagnosed  the  oral  tentacles  qr/x 


\ 


-34 


THE  PHYLUM   CHORDATA  35 

cirri  as  ^is,  he  applied  to  it  the  name  of  Branchiostoma  (gill- 
mouthed),  a  name  still  retained  by  experts  in  nomenclature.  The 
name  Amphioxus,  however,  though  given  a  year  or  so  later  by 
Yarrel,  has  been  in  general  use  for  so  long  that  it  will  be  difficult 
to  displace. 

Amphioxus  has  been  studied  extensively  for  over  a  century  and 
few  details  of  its  structure  or  development  have  escaped  analysis. 
It  has  come  to  be  rather  generally  believed,  following  Willey,  that 
this  form,  "though  specialized  in  some  particulars  and  degenerate 
in  others,  represents  a  grade  of  organization  not  far  removed  from  that 
of  the  main  line  of  early  chordate  ancestors."  Whether  Amphioxus 
is  primitively  simple  or  secondarily  simplified  by  degeneration,  the 
fact  remains  that  in  its  structure  and  development  it  shows  in  a 
strikingly  diagrammatic  way  the  essential  characters  of  the  chordates. 
On  this  account  Amphioxus  has  become  a  favorite  laboratory  type 
throughout  the  educational  institutions  of  the  civilized  world  and  is 
studied  annually  by  thousands  of  students. 

The  Cephalochordata  consist  of  several  closely  similar  speclos 
grouped  into  two  genera,  Branchiostoma  (Amphioxus)  and  Asymme- 
tron.  The  group  is  cosmopolitan  in  distribution,  occurring  almost 
everywhere  in  the  temperate  zone  where  sloping  sandy  sea-shores 
exist.  This  wide  distribition  and  slight  variability  are  taken  by  some 
writers  to  mean  that  the  group  is  extremely  archaic. 

The  writer  is  inclined  to  look  upon  Amphioxus  as  a  form  which  has 
lost  through  sedentary  life  most  of  its  head  parts  .and  is  therefore 
partially  acephalic,  a  view  that  is  based  on  the  following  considera- 
tions. Typically,  the  chordate  notochord  runs  only  up  to  the  head 
proper,  and  the  fact  that  in  Amphioxus  the  notochord  extends  to  the 
anterior  end  of  the  body  (Fig.  8)  may  mean  that  the  head  is  degener- 
ate — has  retreated  from  the  anterior  end.~~This  interpretation  of  Am- 
phioxus  is  in  accord  with  the  fact  that  the  tunicate  larva  has  a  much 
better  head  than  has  Amphioxus.  Strictly  speaking,  Amphioxus  is 
not  headless;  it  has  merely  a  reduced  or  degenerate  head  which  does 
not  extend  in  front  of  the  trunk,  but  has  come  to  lie  back  of  the  most 
anterior  parts  of  the  trunk.  In  the  tunicates  also  the  reduced 
brain  lies  posterior  to  the  mouth  and  parts  of  the  pharynx.  The 
ancestral  Amphioxus  probably  had  a  head  with  paired  eyes,  otic 
vesicles,  and  a  considerably  larger  brain  than  is  found  in  the  modern 
representatives.  Possibly  also  the  pharynx  is  more  specialized  than 


36 


VERTEBRATE  ZOOLOGY 


primitive,  in  that  it  has  such  a  very  large  number  of  pharj  igeal  clefts. 
In  other  respects  Amphioxus  may  be  accepted  as  an  approximate 
prototype  of  the  ancestral  chordate. 


SESSILE  LIFE  AND  METHOD  OF  FEEDING  OF  AMPHIOXUS 

Along  the  sandy  shores  of  the  Mediterranean  and  other  temperate 
seas  the  "lancelet"  leads  a  semi-sedentary  life,  burrowing  rapidly 


FIG.  9. — A  group  of  lancelets  (Amphioxus  lanceolatus}  in  normal  habitat,  some 
in  the  sedentary  position  with  only  the  anterior  end  protruding  from  the  sand 
burrow,  one  in  the  foreground  beginning  to  dig  a  new  burrow,  and  others  swim- 
ming about  in  fish-like  fashion.  (Redrawn  from  indistinct  photograph  after 
Wffley.) 

head  first  in  the  sand,  leaving  the  head  end  protruding  (Fig.  9)  with 
its  oral  hood  and  buccal  tentacles  outspread  to  test  and  draw  in  the 
"sea-soup,"  as  the  food-laden  water  has  been  called.  From  time  to 
time  the  burrow  is  left  and  another  made.  The  method  of  food  gather- 
ing is  essentially  that  of  a  sedentary  organism.  There  is  no  active 
searching  for  food,  but  the  water  which  is  swept  in  quantities  through 
the  pharynx  and  down  through  the  pharyngeal  clefts  gives  up  its  mi- 


THE  PHYLUM   CHORDATA 


37 


nc 


nute  organic  particles,  such  as  protozoa  and  pelagic  larvae.  The 
mechanism  for  concentrating  this  dilute  food  is  somewhat  complex. 
The  pharynx  is  lined  with  ciliated  epithelium  and  the  cilia  beat 
inward  so  as  to 
cause  a  current  of 
water  to  be  drawn 
into  the  mouth  and 
out  through  the 
gill-slits.  The  cilia 
beat  also  down- 
ward, so  as  to  force 
the  current  to  the 
floor  of  the  phar- 
ynx, where  there  is 
a  hypopharyngeal 
groove  or  endostyle  J 
(Fig.  10)  fiflecf  with 
sticky  mucus  to 
which  food  parti- 
cles adhere.  The 
endostyle  is  pro- 
vided with  strong 
cilia  which  whip 
the  mucus  into  a 
rope-like  mass  and 
drive  it  forward  en- 
with  its  burden  of 
food  particles  to 
~ihe  anterior  end 
of  the  pharnyx. 
There  the  endo- 
style bifurcates 
around  the  mouth 


mf 


FIG.  10. — Transverse  section  through  the  pharyngeal 
region  of  Amphioxus.  a,  atrial  cavity;  c,  ccelomic  cav- 
ity; df,  dorsal  fin  fold;  en,  endostyle;  fr,  *fin  ray;  I,  liver 
diver ticulum;  m,  myotome;  me,  myocomma;  mf,  meta- 
pleural  fold;  n,  notochord;  nc,  nerve  cord  or  spinal 
chord;  ne,  nephridium;  p,  pharynx;  pc,  pharyngeal  cleft; 
sg,  supra-pharyngeal  or  dorsal  groove.  (Redrawn  and 
modified  after  Lankester  and  Boveri.) 


in     the     form     of 

two       semicircular 

grooves,    the    peri- 

pharyngeal  bands,  which  unite  again   above  the  mouth  and  form 

the  dorsal  or  hyperpharngeal  groove  (Fig.  10) .   The  mucous  rope  travels 

backward  in  the  dorsal  groove  till  it  reaches  the  stomach  and  intes- 


38  VERTEBRATE  ZOOLOGY 

testine.    There  its  load  of  food-particles  is  digested  and  the  mucus 
itself  passes  out  of  the  anus. 

It  is  of  considerable  interest  to  note  in  this  connection  that  this 
complex  food-concentrating  mechanism  is  found  in  the  urochordates 
and  in  the  Ammocoetes  larva  of  the  lamprey  eels,  but  nowhere  else  in 
the  animal  kingdom.  Thus  it  furnishes  a  point  of  connection  be- 
tween Amphioxus  and  the  lower  chordates,  on  the  one  hand,  and  be- 
tween Amphioxus  and  the  vertebrates,  on  the  other.  It  shouldjilso^ 
bejipted  that  the  primary  Junction  of  the  gharyngeal  apparatus,  in- 
cluding  pjiai7mp;Agii]  ^ft-g,  appears  to  be  that  of  foojjc^n^euEraTign 
rather  tnan  that  of  respiration. 

"      ? 
CHARACTERS  OF  AMPHIOXUS 

1.  Characters  Associated  with  the  Reduced  Head. — The  brain 
is  extremely  small,  hardly  as  large  in  diameter  as  the  rest  of  the  neural 
tube,  (Figs.  11,  A  and  B).     There  are  but  two  pairs  of  cranial  nerves 
which  have  been  called  olfactory  and  optic;  but  in  so  reduced  a  brain 
homologies  are  uncertain.    The  sense  organs  consist  of  a  median  ol- 
factory funnel  opening  into  the  neuroccel,  a  median  rudimentary  eye- 
spot  on  the  anterior  end  of  the  brain,  representing  probably  the  last 
rudiments  of  the  ancestral  paired  eyes.    The  notochord  extends  the 
entire  length  of  the  body,  projecting  in  front  of  the  brain.    This  may 
mean  that  the  brain  has  retreated  from  a  primitive  anterior  position. 
There  is  no  cranium.    Possibly  the  ancestral  chordate  had  some  sort 
of  cranium. 

2.  Characters  that  make  up  the  Food-Concentrating  Mechanism. — 
Since  the  method  of  feeding  has  been  described  these  characters  need 
only  be  listed.     The  mouth  is  an  oral  funnel  bounded  by  ciliated 
buccal  tentacles  with  cartilaginous  supports  that  serve  to  funnel  the 
water  into  the  pharynx.    Separating  the  oral  funnel  from  the  pharynx 
is  a  velum  composedyrf  a  membrane  with  sphincter  muscles  and  a  set 
of  velar  tentaclesfihat  serve  as  a  grating  and  strain  out  the  larger  par- 
ticles.   TheQjmirnyx  has  sometimes  upwards  of  fifty  pairs  of  clefts 
that  are  sepaTated^by  partitions  in  which  lie  cartilaginous  skeletal 
rods  connected  across  with  one  another,  forming  a  sort  of  branchial 
basket.    The  endostyle,  peripharyngeal,  and  hyperpharyngeal  grooves 
secrete  mucus  and  propel  the  food  to  the  stomach  by  means  of  the 
mucous  rope  food  carrier.    The  atrium  Is  a  sort  of  mantle  composed  of 
folds  of  the  body  wall  that  incloses  the  whole  branchial  apparatus  in 


THE  PHYLUM   CHORDATA 


39 


a  voluminous  water-filled  chamber,  the  atrial  cavity.  The  atrium  is 
lined  with  ectoderm  and  has  but  one  opening  to  the  exterior,  a  poste- 
riorly directed  atriopore,  which  carries  off  the  water  that  comes 
through  the  pharyngeal  clefts.  The  atrium  is  a  protection  for  the 
jelicate  pharynx  while  the  animal  is  in  its  sandy  burrow  and  helps 


B 


aes 


FIG.  11.— A.  Lateral  view  of  brain  of  Amphioxus.  cv,  central  vesicle;  dd,  dorsal 
dilatation  of  the  neural  canal;  es,  eye  spot;  of,  olfactory  funnel;  np,  neuropore; 
/,  first  cranial  nerve,  olfactory;  II,  second  (optic)  cranial  nerve,  showing  dorsal 
and  ventral  roots. 

B.  Dorsal  view  of  brain  and  spinal  cord  of  Amphioxus.  aes,  accessory  dorsal 
•  eye  spots,  some  median,  some  paired,  es,  eye  spot;  /  and  //,  first  and  second 
cranial  nerves;  sn,  spinal  nerves.  (Redrawn  from  Willey,  after  Hatschek  and 
Schneider.) 

3.  General  Characters.— The  alimentary  system  consists  of  the 
harnyx,  a  short,  straight  stomach  intestine  terminating  in  a  ventral 
onuspwhich  opens  to  the  left  of  the  ventral  fin.  The  stomach  gives 
off  a  ventral  diverticulum  or  liver,  which  is  directed  forward. 

The  circulatory  system  (Fig.  14)  consists  of  a  ventral  pulsating  vessel 
with  no  specialized  heart  enlargement,  which  pumps  the  colorless 


40 


VERTEBRATE  ZOOLOGY 


tb. 


"c 


"Sfc. 


FIG.  12. — Transverse  section  of  the  ventral  part  of  the  pharynx  of  Amphioxus. 
c,  co?lom;  e,  endostyle;  gl,  endostyjar  glands;  mba,  median  branchial  artery;  pb, 
primary  bar;  sk,  endostylar  and  branchial  rods  and  skeletal  plates;  tb,  tongue  bar. 
(From  Herdman,  after  Lankester.) 


FIG.  13. — Diagrammatic  transverse  sections  of  Amphioxus  to  show  three  stages 
(A,  B,  C)  in  the  development  of  the  atrium,  ao,  aorta;  c,  dermis;  d,  intestine; 
/,  fascia;  fh,  cavity  for  dorsal  fin  ray;  m,  myomere;  n,  neural  tube;  p,  atrium; 
sfh,  metapleural  folds;  si,  subintestinal  vein;  sk,  sheath  "of  notochord  and  neural 
tube;  si,  sub-atrial  ridge;  sp,  coelom.  (From  Parker  and  Haswell,  after  Lankester 
and  Willey.) 


THE  PHYLUM   CHORDATA 


41 


blood  forward  and  through  the  branchial  arches,  where  it  is  aerated. 
The  blood  collects  again  in  paired  dorsal  aortce,  which  unite  back  of 

eba  da 


pc 


va 


FIG.  14. — Diagram  of  the  circulatory  system  of  Amphioxus.  aba,  afferent 
branchial  artery;  d,  liver  diverticulum;  da,  dorsal  aortse;  e,  \efferent  branchial 
arteries;  hp,  hepatic  portal  system;  pc,  pharyngeal  clefts;  ph,  pharynx;  sv,  sub- 
intestinal  vein;  va,  ventral  aorta.  .  (Modified  after  Parker  and  Haswell.) 

the  pharnyx  into  a  single  dorsal  aorta,  that  in  turn  carries  the  blood 
to  the  various  systems.  The  ventral  vessel  takes  a  loop  about  the 
diverticulum  and  this  loop  is  interpreted  as  a  simple  hepatic  portal 

system. 


A  B 

FIG.  15. — A.  Nephridium  of  Amphioxus  with  incurrent  funnels  crowned  with 
solenocytes.  B.  Enlarged  view  of  a  portion  of  one  nephridial  funnel,  showing 
solenocytes.  (After  Boveri  and  Goodrich.) 

The  excretory  system  consists  of  paired  nephridia  (Fig.  15,  A)  with 
ciliated  nephrostomes  from  which  protrude  knobbed  cells  called  soleno- 
cytes (Fig.  15,  B).  The  nephridia  are  true  ccelomoducts,  leading  from 


42  VERTEBRATE  ZOOLOGY 

the  greatly  reduced  coelom  to  the  atrium.  They  are  segmental  organs 
and  occur  typically  a  pair  to  a  metamere  in  the  pharyngeal  region. 
The  resemblance  between  these  structures  and  those  of  annelids  is 
fairly  close.  The  muscular  system  consists  of  segmental  myotomes 
separated  by  connective  tissue  myocommata.  The  myotomes  are 
chevron-shaped  as  seen  from  the  side,  and  resemble  those  of  the 
fishes. 

The  spinal  cord  has  spinal  nerves  (Fig.  11)  that  alternate  on  the 
two  sides  and  have  dorsal  and  ventral  roots.  The  fin  system  is  very 
primitive,  consisting  of  a  low  continuous  median  fin-fold  running  un- 
interruptedly about  the  tail  and  ending  back  of  the  atriopore.  The 
fin-folds  are  supported  by  short  connective  tissue  fin-rays.  Paired 
ridges,  called  metapleural  folds,  run  along  the  ventro-lat'eral  portions 
of  the  body;  they  have  been  thought  of  as  the  primordia  of  paired  ap- 
pendages. The  integument  consists  of  a  single  layer  of  ectodermal 
cells  and  several  layers  of  dermal  cells. 

The  gonads  are  simply  metameric  pouches  of  the  co3lom  hi  the  bran- 
chial region.  The  eggs  and  sperm  escape  by  rupture  of  the  body 
wall  into  the  atrium,  and  fertilization  is  external.  The  sexes  are  sep- 
arate. The  egg  is  small  and  practically  yokeless. 

EMBRYOLOGY  OF  AMPHIOXUS 

"As  an  introduction  to  the  study  of  embryology,  and  as  an 
indispensable  aid  to  a  reasonable  appreciation  of  the  value  of  em- 
bryological  facts,  the  life-history  of  Amphioxus  provides  an  ob- 
ject, which  for  its  capacity  of  application  to  almost  eveiy  branch 
of  zoological  discussion  is  perhaps  unrivaled.  All  the  funda- 
mental structures  of  the  body  are  laid  down  with  schematic 
clearness."  (Willey.) 

The  ovum  is  microscopic  and  resembles  those  of  many  marine  in- 
vertebrates. Spawning;  occurs  at  sun-down  when  simultaneously 
females  and  males  discharge  ova  and  spermatozoa  into  the  sea.-wa.tffl;. 
where  fertilization  occurs^.  Maturation  phenomena  resemble  those  of 
invertebrates,  as  do  also  the  cleavage  stages,  the  first  two  cleavages 
being  from  pole  to  pole  (meridional)  and  the  third  equatorial,  pro- 
ducing a  tetrad  of  micromeres  and  a  tetrad  of  macromeres  (Fig.  16). 
The  micromere  cells  divide  more  rapidly  than  the  macromere  cells  and 
a  blastula  (Fig.  16,  F)  is  formed  with  smaller  ectodermal  cells  at  the 


THE  PHYLUM  CHORDATA 


43 


animal  pole  and  larger  endodermal  cells  at  the  vegetal  V)le.  The 
endoderm  cells  soon  cave  into  the  dome  of  ectoderm  cells  and/orm  an 
incipient  gastrula  as  in  Fig.  17,  A,  B,  C.  This  invagination  continues 
until  a  typical  embolic  gastrula  is  formed  as  in  Figs.  17,  D  and  3$.  A 


FIG.  16. — Cleavage  of  Amphioxus.  A.  Four-cell  stage  viewed  from  the  animal 
pole.  The  two  antero-dorsal  cells  are  the  smaller.  B.  Eight-cell  stage  viewed 
from  the  animal  pole  showing  the  four  sizes  of  cells.  C.  Sixteen-cells  viewed 
from  the  left  side.  D.  Thirty-two  cells  viewed  from  the  vegetal  pole.  E.  Thirty- 
two  passing  into  sixty-four  cells,  viewed  from  the  antero-dorsal  region.  F.  Optical 
section  of  right  half  of  young  blastula.  About  128  cells,  a,  animal  pole;  ad, 
antero-dorsal;  I,  left;  pv,  posterior  ventral;  r,  right;  v,  vegetal  pole.  (From 
Kellicott's  "Outlines  of  Chordate  Development"  [Henry  Holt  and  Company] 
after  Cerfontaine.) 


FIG.  17. — Gastrulation  of  Amphioxus.  A.  Blastula  showing  flattening  of  the 
vegetal  pole  and  rapid  proliferation  of  cells  in  the  posterior  region  (germ  ring). 
B.  Flattening  more  pronounced;  mitosis  in  cells  of  germ  ring.  C.  Commencement 
of  the  infolding  (invagination)  of  the  cells  of  the  vegetal  pole.  D.  Continued  in- 
folding, and  inflection,  or  involution,  of  ectoderm  cells  in  the  dorsal  lip  of  the 
blastopore.  The  blastoccel  becoming  obliterated  and  the  archenteron  being  estab- 
lished. E.  Invagination  complete.  Continued  involution  of  the  dorsal  lip  of 
blastopore.  Mitoses  in  germ  ring.  F.  Constriction  of  blastopore  and  commence- 
ment of  elongation  of  the  gastrula.  Remnants  of  blastocool  in  ventral  lip  of 
blastopore.  H.  Neurenteric  canal  established  by  overgrowth  of  neural  folds. 
Continued  mitosis  in  germ  ring,  a,  animal  pole;  ar,  archenteron;  6,  blastoporal 
opening;  ch,  rudiment  of  notochord;  dl,  dorsal  lip  of  blastopore;  ec,  ectoderm; 
en,  endoderm;  gr,  germ  ring;  nc,  neurenteric  canal;  nf,  neural  fold;  np,  neural 
plate;  s,  blastoco?!  or  segmentation  cavity;  v,  vegetal  pole;  vl,  ventral  lip  of 
blastopore;  //,  second  polar  body.  (From  Kellicott's  "  Outlines  of  Chordate 
Development "  [Henry  Holt  and  Company]  after  Cerfontaine.) 

44 


np 


FIG.  18. — Transverse  sections  through  young  embryos  of  Amphioxus,  showing 
formation  of  nerve  cord,  notochord,  and  mesoderm.  A.  Commencement  of  the 
growth  of  the  superficial  ectoderm  (neural  folds)  above  the  neural  plate  (medullary 
plate).  B.  Continued  growth  of  the  ectoderm  over  the  neural  plate.  Differentia- 
tion of  the  notochord,  and  first  indications  of  mesoderm  and  enteroccelic  cavities. 
C.  Section  through  middle  of  larva  with  two  somites.  Neural  plate  folding 
into  tube.  D.  Section  through  first  pair  of  mesodermal  somites  now  completely 
constricted  off.  E.  Section  through  middle  of  larva  with  nine  pairs  of  somites. 
Neural  plate  folded  into  a  tube.  Notochord  completely  separated.  In  the  inner 
cells  of  the  somites  muscle  fibrillae  are  forming,  ar,  archenteron;  c,  enterocoel; 
ch,  notochord;  ec,  ectoderm;  en,  endoderm;  /,  muscle  fibrillse;  g,  gut  cavity;  m, 
unsegmented  mesoderm  fold;  ms,  mesodermal  somite;  nc,  neuroccel;  nf,  neural 
fold;  np,  neural  plate;  nl,  neural  tube.  (From  Kellicott's,  "  Outlines  of  Chordate 
Development"  [Henry  Holt  and  Company]  after  Cerfontaine.) 

45 


46 


VERTEBRATE  ZOOLOGY 


late  gastrula  shows  a  distinct  bilaterality,  a  dorsal  and  ventral  aspect, 
and  anterior  and  posterior  lips  to  the  blastopore.    The  gastrula  is  a 

ch 


rd 


FIG.  19. — Optical  section  of  young  embryos  of  Amphioxus.  The  cilia  are 
omitted.  A.  Two-somite  stage,  approximately  at  the  time  of  hatching,  showing 
relation  of  neuropore  and  neurenteric  canal.  B.  Nine-somite  stage,  showing 
origin  of  anterior  gut  diverticula.  C.  Fifteen-somite  stage.  End  of  the  embryonic 
period,  ap,  anterior  process  of  first  somite;  c,  neurenteric  canal;  ch,  notochord; 
ego,  external  opening  of  club-shaped  gland;  co,  ccelomic  cavity  of  somite;  cv, 
cerebral  vesicle;  g,  gut  cavity;  gs,  rudiment  of  first  gill  slit;  i,  intestine;  I,  left 
anterior  gut  diverticulum;  m,  mouth;  mes,  unsegmented  mesoderm;  n,  nerve 
cord;  p,  pigment  spot  (eye  spot);  rd,  right  anterior  gut  diverticulum;  si,  s2,  first 
and  second  mesodermal  somites;  spc,  splanchnocoel  (body  cavity).  (From 
Kellicott's  "  Outlines  of  Chordate  Development "  [Henry  Holt  and  Company] 
after  Hatschek.) 


ciliated  embryo  that  moves  about  within  its  vitelline  membrane  very 
much  like  that  of  an  echinoderm  or  annelid.    After  about  eight  hours 


THE   PHYLUM   CHORDATA 


47 


the  gastrula  emerges  from  the  vitelline  membrane  as  a  free-swimming 
larva.  It  is  now  an  elongated  gastrula  and  has  begun  to  develop  the 
rudiments  of  definitive 
structures,  such  as  noto- 
cbord,  coelom,  and  med- 
ullary plate.  The  first 
signs  of  metamerism  are 
seen  in  connection  with 
a  series  of  paired  lateral 
pouches  derived  from 
the  archenteron,  (Fig. 
19  A)  beginning  near  the 
anterior  end  and  pro- 
ceeding posteriorly  un- 
til fourteen  pairs  are 
formed.  Additional  seg- 
ments arise  as  a  direct 
outgrowth  of  the  hind 
end  of  the  body. 

The  medullary  plate 
(Fig.  18)  is  cut  off  from 
the  ectoderm  of  the 
body  wall  in  a  peculiar 
way  by  the  cooperation 
of  two  factors.  The 
ectodermal  ridges  at  the 
side  of  the  plate  arch 
over  the  middle  parts  of 
the  plate  and  the  ecto- 
derm of  the  ventral  lip 
of  the  blastopore  grows 
like  a  sheet  over  the 
blastopore  and  closes 
the  latter  so  that  instead 
of  opening  to  the  outside 
it  communicates  with 


FIG.  20. — Sections  through  young  Amphioxus 
embryos  showing  the  origin  of  the  anterior  gut 
diverticula.  A.  Frontal  section  through  embryo 
with  nine  pairs  of  somites.  (See  Fig.  19,  B).  The 
dotted  line  marks  the  course  of  the  gut  wall  ven- 
tral to  the  level  of  the  section.  B.  Optical  sagit- 
tal section  through  anterior  end  of  embryo  with 
thirteen  pairs  of  somites  showing  position  of  right 
anterior  gut  diverticulum.  C.  Same  in  ventral 
view,  c,  coalomic  cavity  of  somite;  ch,  notochord; 
csg,  rudiment  of  club-shaped  gland;  d,  rudiment  of 
anterior  gut  diverticula;  ec,  ectoderm;  en,  endo- 
derm;  g,  gut  cavity;  gsl,  rudiment  of  first  gill  slit; 
Id,  left  anterior  gut  diverticulum;  n,  nerve  cord; 
np,  neuropore;  rd,  right  anterior  gut  diverticulum; 
«i,  «2,  89,  first,  second,  ninth  mesodermal  somites. 
(From  Kellicott's  "  Outline  of  Chordate  Develop- 
ment "  [Henry  Holt  and  Co.]  after  Hatschek.) 


the  neural  tube  and  has  become  a  neur enteric  canal,  the  homologue  of 
which  is  found  in  all  vertebrates  and  constitutes  one  of  the  most 
peculiar  characters  of  the  group. 


48  VERTEBRATE  ZOOLOGY 

A  detailed  study  of  the  organogeny  of  Amphioxus  would  not  fall 
within  the  scope  of  the  present  volume,  but  brief  mention  should  be 
made  of  a  few  of  the  later  stages,  and  of  the  more  significant  changes 
involved.  A  stage  of  much  interest  is  shown  in  Fig.  20.  This  is  an 
embryo  showing  nine  pairs  of  primitive  somites  and  in  process  of 
budding  off  the  right  and  left  head  cavities.  Up  to  this  point  the 
embryo  is  strictly  bilaterally  symmetrical.  The  first  disturbance  of 
bilaterality  is  seen  in  connection  with  the  head-cavities,  for  the  right 
one  grows  large  and  becomes  the  cavity  of  the  snout  or  pre-oral  body 
cavity,  lying  beneath  the  notochord,  and  the  left  becomes  a  rudimen- 
tar}r  structure,  the  pre-oral  pit.  In  Fig.  20,  C  is  shown  the  beginning  of 
the  overgrowth  of  the  right  head  cavity.  This  crowding  over  of  the 
right  side  of  the  head  toward  the  left  disturbs  the  primitive  location 
of  the  mouth,  so  that  it  opens  to  the  left  side  and  only  secondarily 
adjusts  itself  to  the  nearly  median  postition  characteristic  of  the 
adult.  The  gill-slits,  myotomes,  spinal  nerves,  nephridia,  and  other 
metameric  structures  are  thrown  out  of  the  primitive  paired  arrange- 
ment and  alternate  on  the  two  sides  throughout  life.  In  the  genus 
Asymmetron  the  asymmetry  of  the  whole  body  is  more  pronounced 
than  in  the  more  typical  members  of  the  Amphioxus  family;  hence 
the  name.  This  twisting  about  of  the  body  is  a  characteristic  of 
sessile  animals  and  it  is  significant  to  find  in  Amphioxus  a  condition 
comparable  with  that  seen  in  the  echinoderms,  which  are  entirely 
headless  creatures,  and  in  Balanoglossus  in  which  the  head  is  greatly 
reduced. 

More  advanced  larvae  of  Amphioxus  show  the  migration  of  the 
mouth  from  the  left  side  to  the  middle,  the  method  of  uniting  the 
primary  gill-slits  into  the  definitive  double  type  separated  by  tongue 
bars,  the  formation  of  the  oral  hood  and  buccal  cirri.  There  is  no  sud- 
den change  equivalent  to  metamorphosis,  but  at  this  time  the  larva 
gives  up  its  free-swimming  habit  and  begins  to  lead  the  semi-seden- 
tary, sand-burrowing  life  of  the  adult.  The  period  of  adolescence  is 
long  and  slow  and  culminates  in  the  maturing  of  the  gonads. 

SUB-PHYLUM  II.     UROCHORDATA   (TUNICATES) 

Just  as  there  is  .no  doubt  about  the  affinities  of  Amphioxus  to  the 
vertebrates,  so  there  is  no  question  but  that  the  tunicates  are  related 
to  Amphioxus. 

In  contrast  with  the  cephalochordates,  in  which  one  family  of  two 


THE   PHYLUM   CHORDATA 


49 


genera  and  only  a  few  species  exist,  the  urochordates  constitute  a 
large  assemblage  of  highly  diverse  forms,  ranging  from  purely  pelagic 
to  purely  sessile  forms,  and  from  solitary  forms  of  large  size  to  colo- 


Ascidiids 
Mol  guilds. 


pIG  21. — Sketch  of  the  chief  kinds  of  Urochordata  found  in  the  sea  showing 
their  distribution  and  habits.  The  dotted  lines  on  the  left  indicate  the  life  zones 
of  the  sea:  the  surface  or  pelagic  zone;  the  middle  zone;  and  the  sea-bottom  zone. 
(From  Herdman  in  the  Cambridge  Natural  History,  Vol.  VII.) 

nial  forms  of  comparatively  small  size.  They  are  plentiful  all  over  the 
oceans  from  the  shallow  shore  waters  to  the  deepest  abysses.  By  far 
the  majority  of  them  are  sessile  in  habit  in  adult  life,  remaining 


50 


VERTEBRATE  ZOOLOGY 


permanently  rooted  to  one  spot.  Some  of  the  free-swimming  forms 
(Larvacea)  are  interpreted  as  paedogenetic  forms,  which  have  retained 
the  larval  condition  throughout  life;  others  are  evidently  derived 
secondarily  from  sessile  ancestors  and  now  live  a  free  pelagic  life. 
The  composite  illustration  (Fig.  21)  shows  the  general  appearance  of 
many  common  types  of  urochordates.  The  sessile  type  seems  to  be 
representative  of,  and  will  serve  to  illustrate,  the  essential  features  of 
the  whole  sub-phylum. 

ORDER  I.  ASCIDIACEA 

A  TYPICAL   ASCIDIAN 

An  external  view  of  an  ascidian  (Fig.  22)  reveals  little  of  interest. 
It  does  not  even  look  like  a  living  creature;  much  less  a  chordate.  It 
is  a  dull  brown  object  resembling  a  leather  bag 
or  bottle  with  two  short  necks,  one  terminal  and 
one  somewhat  on  the  side,  the  former  being  the 
oral  funnel  and  the  latter  the  atrial  funnel  or  atrio- 
pore.  If  one  watches  the  water  currents  care- 
fully he  will  observe  that  the  water  enters  the 
oral  funnel  in  a  steady  stream  and  exits  from 
the  atriopore.  The  wrinkled  brown  covering  or 
tunic  (which  gives  the  name  Tunicata  to  the 
group)  is  merely  a  lifeless  protective  layer  com- 
posed of  animal  cellulose,  a  substance  almost 
identical  with  wood.  The  body  wall  within  the 
tunic  is  composed  largely  of  glands  and  con- 
nective tissue  and  shows  neither  myotomes  nor 
any  other  segmented  structures.  The  tunic  and 
body  wall  of  one  side  removed,  we  get  a  view  of 
the  interior  (Fig.  23).  What  we  find  is  little 


FIG.  22. — External  more  than  an  exaggerated  food-concentrating  ap- 

Sfftaft&fcUrftt!)  Paratm'  closely  comparable  with  that  of  Amphi- 
seen  from  the  right  oxus.     The  circular  oral  funnel  opens  through  a 

side.    (From   Parker  velum,  with  velar  tentacles,  into  a  great  sac-like 
and     Haswell,    after      7  ,-,     ,  f  ,-,          •,       i 

Herdman )  pharynx  that  occupies  far  more  than  its  share 

of  the  available  space.  This  pharynx  is  an 
elaborate  sieve  with  countless  small  slit-like  openings,  stigmata,  which 
are  subdivided  pharyngeal  clefts.  On  the  ventral  side  there  is  an 


THE  PHYLUM   CHORDATA 


51 


af 


endostyle,  around  the  mouth  peripharyngeal  grooves,  and  on  the  dorsal 
side  a  dorsal  lamina  (corresponding  to  the  dorsal  groove  of  Amphioxus) ' 
The  method  of  food  concentration  and  transportation  is  similar  to 
that  of  Amphioxus,  but  the  machine  seems  to  be  of  a  much  improved 
type,  appropriate  for 
purely  sedentary  life. 
An  atrial  cavity  sur- 
rounds the  pharynx, 
which  is  inclosed  by  a 
mantle  that  surrounds 
the  whole  body.  The 
ectoderm  of  this  mantle 
it  is  that  secretes  the  dl 
tunic.  The  atriopore, 
instead  of  being  post- 
erior in  position  and 
backwardly  directed,  is 
close  to  the  mouth  and 
f  orwardly  directed. 
The  stomach  opens  near 
the  bottom  of  the  phar- 
ynx and  the  intestine 
takes  a  complete  turn 
and  opens  forward  into 
the  atrium.  There  is  no 
notochord,  no  neural  tube; 
indeed  almost  none  of 
the  structures  character- 
istic of  the  dorsal  side 
are  present.  A  poor  ex- 
cuse for  a  brain  occurs 
on  the  dorsal  side  of  the 
mouth  embedded  in  the 
body  wall  between  the  two  funnels.  It  is  nothing  but  a  ganglion 
associated  with  a  sub-neural  gland  and  with  a  duct  entering  the  phar- 
ynx. This  last  structure  has  been  compared  to  the  vertebrate  hypo- 
physis. The  ascidian  comes  as  near  being  an  animated  pharynx  as 
could  well  be,  only  certain  absolutely  essential  elements  of  the  other 
parts  of  the  body  being  retained. 


FIG.  23. — Internal  anatomy  of  a  typical  Asci- 
dian. a,  atrial  cavity;  af,  atrial  funnel  or  atrio- 
pore; an,  anus;  dl,  dorsal  lamina;  e,  endostyle; 
g,  gonad;  gd,  duct  of  gonad;  h,  heart;  hy,  hypo- 
physeal  duct;  i,  intestine;  m,  mantle,  ng,  neural 
gland;  oe,  oesophagus;  of,  oral  funnel  or  mouth; 
s,  stomach;  st.  stigmata  or  subdivided  pharyngeal 
clefts;  v,  velum.  (Considerably  modified  after 
Hertwig.) 


52  VERTEBRATE  ZOOLOGY 

The  heart  is  no  less  than  an  oddity.  It  is  a  pulsating  tube,  lying 
ventral  to  the  stomach,  open  at  both  ends  into  sinuses.  It  works  by 
peristaltic  contractions  first  in  one  direction  for  a  few  beats  and  then 
in  the  other.  The  colorless  blood  is  thus  driven  through  sinuses  first 
toward  the  anterior  and  then  toward  the  posterior  organs.  No  other 
such  heart  is  known  among  animals. 

All  ascidians  are  without  exception  hermaphroditic,  as  befits  their 
sessile  life,  though  there  is  a  high  degree  of  self-sterility.  Usually 
the  eggs  of  one  individual  meet  the  sperm  of  another  in  the  sea-water. 

An  adult  ascidian  does  not  therefore  appear  to  be  an  impressive 
candidate  for  the  honor  of  being  classed  as  a  chordate,  but  it  makes  a 
much  better  showing  when  we  consider  the  entire  ontogeny. 

DEVELOPMENT  AND  METAMORPHOSIS  OF  AN  ASCIDIAN 

The  ascidian  egg  is  small  and  almost  yolkless.  The  cleavage 
stages,  blastula,  and  early  gastrula  are  essentially  similar  to  those  of 
Amphioxus,  except  that  the  embryonic  life  is  longer.  The  gastrula 
is  not  a  ciliated  larva  but  a  passive  embryo;  in  fact  the  embryonic 
development  is  greatly  prolonged  and  the  larval  life  does  not  com- 
mence unlil  the  egg  hatches  and  the  advanced  tadpole  larva  emerges. 


czdh. 


FIG.  24. — Anterior  end  of  the  "tadpole"  larva  of  an  ascidian  (Astidia  mam 
millata)  in  optical  section,  adh,  adhesive  papillae;  ali,  rudimentary  alimentary 
tract;  atr,  atrial  aperture;  til.  gr,  ciliated  diverticulum,  becoming  ciliated  funnel; 
end,  endostyle;  eye,  eye;  med,  nerve-cord;  noio,  notochord;  oto,  otocyst;  sens,  ves, 
sense- vesicle  (cavity  of  brain) ;  stig,  earliest  stigmata  or  pharyngeal  clefts.  (From 
Parker  and  Haswell,  after  Kowalevsky.) 

The  Ascidian  Tadpole. — When  we  examine  the  larva  of  the  tunicate 
(Fig.  24)  we  understand  why  the  Urochordata  have  been  classed  with 
the  chordates.  The  notochord  is  quite  typical  but  extends  only  as 
far  as  the  pharynx,  or,  as  the  name  Urochordata  implies,  is  confined  to 
the  tail.  The  tail  is  also  characterized  by  having  dorsal  and  ventral 
fins  and  a  neural  tube.  The  brain  or  sense  vesicle  is  much  larger  than 


THE  PHYLUM  CHORDATA  53 

the  neural  tube  and  is  much  more  like  a  vertebrate  brain  than  is  that 
of  Amphioxus,  since  it  has  connected  with  it  paired  optic  vesicles  and 
otocysts.  The  tadpole  larva  therefore  makes  up  for  the  deficiencies 
of  the  adult  in  having  a  good  notochord  and  a  neural  tube.  It  is, 
however,  embryonic  in  the  pharynx  and  alimentary  tract.  At  first 
there  is  no  mouth  and  no  anal  opening  and  the  pharynx  has  only  a 
very  few  clefts.  The  atrium  and  atriopore  also  are  at  first  absent. 

Larval  Metamorphosis. — The  tadpole  larva  leads  an  active  free- 
swimming  life  for  a  short  time  and  then  settles  down  upon  the  bottom 
or  upon  some  other  solid  object.  Attachment  is  secured  by  means  of 
three  sucker-like  papillae  or  "  chin-warts  "  and  the  larva  undergoes  a 
very  rapid  metamorphosis.  All  of  the  structures  pertaining  to  a  free 
active  life  atrophy  and  the  food-concentrating  apparatus  becomes 
greatly  elaborated.  Tail,  notochord,  and  neural  tube  disappear.  The 
brain  degenerates  to  a  ganglion.  While  the  tail  is  atrophying  the  chin 
or  ventral  side  of  the  pharynx  grows  out  of  all  proportion  to  the  rest  of 
the  body  so  that  the  mouth  is  pushed  away  from  the  point  of  attach- 
ment and  comes  to  be  at  the  free  end.  The  dorsal  side  of  the  pharyn- 
geal  region  diminishes  rapidly  also,  so  as  to  cause  the  stomach  and 
intestine  to  turn  upward  into  a  U-shaped  body,  the  anus  opening  in 
the  same  direction  as  the  mouth.  The  atrium  arises  as  two  ectodermal 
invaginations  on  right  and  left  sides.  These  remain  separate  for 
some  time,  but,  after  they  have  grown  in  so  as  to  surround  the  whole 
pharynx,  the  two  cavities  fuse  and  lose  the  bounding  wall  in  the 
dorsal  side,  but  remain  separate  at  the  median  ventral  line  near  the 
endostyle.  There  is  therefore  not  much  resemblance,  so  far  as  mode 
of  origin  is  concerned,  between  the  atrium  of  a  tunicate  and  that  of 
Amphioxus.  The  one  point  in  common  is  that  it  is  lined  with  ecto- 
derm. 

The  tadpole  larva  is  a  very  good  chordate  larva,  quite  comparable 
in  most  respects  to  a  vertebrate  larva,  such  as  that  of  a  fish  or  a  frog 
tadpole;  but  the  adult  ascidian  is  a  lowly  creature  with  an  organization 
not  much  higher  than  that  of  a  coelenterate  or  at  best,  a  worm.  It  is 
quite  evident  then  that  the  ascidian  presents  a  typical  case  of  degen- 
eration accompanying  the  assumption  of  sessile  life.  The  well-devel- 
oped brain  of  the  ascidian  larva  indicates  that  the  ancestral  chordate 
had  a  better  brain  and. Jiead.  than  has_AmphiQX]il3,  and  had  an  organ-. 
izaiiQ.n  not  far  from  intermediate  between  thatjrf  the  adult Amphioxus 
and  that  of  Jbhe  ascidian  larva. 


54 


VERTEBRATE  ZOOLOGY 


FIG.  25. — Diagram  showing  the  metamorphosis  of  the  free-swimming  "tad- 
pole" larva  of  an  Ascidian  into  the  sessile  or  fixed  adult  condition.  A,  stage  of 
free-swimming  larva;  B,  larva  recently  fixed;  C,  older  fixed  stage,  adh,  adhesive 
papillae;  atr,  atrial  cavity;  cil.  gr,  ciliated  diverticulum,  becoming  ciliated  funnel; 
end,  endostyle;  ht,  heart;  med,  nerve  cord  of  trunk;  n.  gn,  nerve  ganglion;  noio, 
notochord;  or,  oral  aperture;  rect,  rectum;  sens.ves,  sense  vesicle  or  brain;  stig, 
stigmata;  stol,  stolon;  t,  tail.  (From  Parker  and  Haswell,  after  Seeliger.) 


THE  PHYLUM   CHORDATA 


55 


Colonial  Ascidians  (Ascidiae  Compositse). — All  of  the  colonial 
ascidians  are  produced  by  budding  of  a  single  parent  individual.  It  is 
usual  to  find  a  group  of  ascidio- 
zooids  grouped  together  in  such  a 
way  that  they  have  a  common 
cloacal  opening  (Figs.  26  and  27). 
Naturally  also  the  tunics  of  the 
various  individuals  form  a  common 
matrix  in  which  they  appear  to  be 
embedded.  The  so-called  "sea- 
pork"  from  our  Atlantic  coast  is  a 
typical  example  of  that  fixed  type 
of  colonial  ascidian,  which  forms 
incrusting  masses  on  rocks  and 
piles  and  gives  the  appearance  of  a 
piece  of  pinkish  fat  pork,  per- 
forated at  intervals  by  groups  of 
pores,  which  are  the  mouths  and 
atrial  pores  of  groups  of  zooids. 

Free-Swimming  Colonies  of 
Ascidians  (Ascidiae  Luciae).— 
These  are  pelagic  forms  in  which  the 
ascidiozooids  are  arranged  in  such 
a  way  that  the  mouth  opening  is  on 
the  outside  of  a  hollow  cylinder 
and  the  atrial  opening  on  the  inside  (Fig.  28).  Since  only  one  end  of 
the  cylinder  is  open,  a  steady  current  of  water  is  forced  out  at  this  end 

c.cl. 


-t. 


FIG.  26. — Surface  view  of  two  sys- 
tems of  colonial  ascidians  of  the  species 
Botryllus  violaceus.  cl,  cloaca  or  com- 
mon atrial  funnel;  or,  oral  funnels  of 
the  individual  zooids.  (From  Herd- 
man,  after  H.  Milne-Edwards.) 


FIG.  27. — Sectional  view  of  a  portion  of  a  colony  of  a  colonial  ascidian,  (Dip- 
losoma)  showing:  c,  cl,  common  cloaca  into  which  atrial  openings  of  all  zooids 
empty;  br,  branchial  apertures  or  oral  funnels  of  individual  zooids;  t,  test  or 
tunic  forming  the  common  matrix  of  the  colony.  (From  Herdman.) 


56 


VERTEBRATE   ZOOLOGY 


and  serves  to  propel  the  colony  through  the  water.     Pyrosoma,  a 
typical  member  of  this  group,  is  found  swimming  near  the  surface  of 


B 


FIG.  28. — The  free-swimming  colonial  ascidian,  Pyrosoma.  A,  lateral  view 
(nat.  size);  B,  view  of  the  open  end;  C,  diagram  of  longitudinal  section;  at,  atrial 
pores  opening  into  the  central  cavity  or  cloaca,  ccl,  of  the  colony;  br,  branchial  or 
oral  apertures  opening  to  the  outside;  end,  endostyle;  t,  test  or  tunic;  v,  velum  or 
diaphragm  at  terminal  opening.  (From  Herdman.) 

warm  seas  and  is  brilliantly  phosphorescent.    Colonies  vary  from  an 
inch  to  upwards  to  twelve  feet  in  length. 

ORDER  II.  TH  ALT  ACE  A  (SALPIANS) 

These  are  all  free-swimming  tunicates  that  live  near  the  sea  surface 
and  are  doubtless  derived  from  the  free-swimming  ascidians.  They 
may  be  either  solitary  or  colonial.  In  the  typical  salpians  the  body 
is  strikingly  like  a  barrel,  open  at  both  ends,  but  with  a  partition 
across  near  the  middle.  The  resemblance  to  a  barrel  is  enhanced  by 
the  presence  of  a  considerable  number  of  muscle  bands  that  encircle 
the  cylinder  like  barrel-hoops.  They  are  semi-transparent  forms, 
often  beautifully  colored,  and  are  capable  of  making  very  good  prog- 
ress by  forcing  water  out  of  the  broad  cloacal  opening  by  contracting 
the  muscles  of  that  region. 

The  structure  is  well  shown  in  the  classic  salpian  type  Doliolwn 
(Fig.  29) .  At  the  left  there  is  the  widely  open  oral  funnel  which  opens 
into  the  broad  sac-like  pharynx.  This  has  a  large  number  of  distinct 
branchial  clefts,  an  endostyle,  and  peripharyngeal  grooves,  but  no  dis- 


THE   PHYLUM   CHORDATA 


57 


tinct  dorsal  lamina.    The  atrium  has  two  wings  that  extend  laterally 
to  receive  water  from  the  pharyngeal  clefts,  and  opens  broadly  on  the 


n.g          a.gl 


at.tn 


FIG.  29. — Individual  of  the  sexual  generation  of  the  Salpian  Doliolum  tritonis 
X  10.  at,  atrial  aperture;  atl,  atrial  lobes;  atm,  wall  of  atrium;  br,  branchial  or 
oral  aperture;  brl,  branchial  lobes  or  tentacles;  brs,  branchial  sac  or  pharynx; 
dt,  dorsal  tubercle;  end,  endostyle;  h,  heart;  m,  mantle;  mi-m8,  circular  muscle 
bands;  n,  nerve;  ng,  nerve  ganglion;  ov,  ovary;  pbr,  peribranchial  or  atrial  cavity; 
pp,  peripharyngeal  bands;  sg,  stigmata;  sgl,  subneural  gland;  so,  sense  organ; 
si,  stomach;  t,  test' or  tunic;  tes,  testis;  z,  prebranchial  zone.  (After  Herdman.) 


FIG.  30. — Life  history  of  Doliolum.  A,  tailed  larval  stage;  B,  "nurse"  or 
oozoiod,  showing  buds  (blastozooids)  migrating  from  the  ventral  stolon  to  the 
dorsal  process;  c,  posterior  part  of  much  later  oozooid  to  show  buds  arranged  in 
three  rows  on  the  dorsal  process;  D,  stolon  segmenting;  E,  young  migrating  bud; 
F,  trophpzooid  developed  from  one  of  the  buds  of  a  lateral  row. 

At,  atrial  aperture;  b,  buds;  Br,  branchial  or  oral  aperture;  cl,  cloaca;  dp,  dorsal 
process;  end,  endostyle;  ht,  heart;  Ib,  lateral  buds;  mb,  median  buds;  ng,  nerve 
ganglion;  ot,  otocyst;  pc,  pericardium;  sk,  stalk;  sto,  stolon.  (From  Herdman, 
after  Uljanin  and  Barrois.) 


58  VERTEBRATE  ZOOLOGY 

right  as  an  atriopore.  The  salpian  is  therefore  not  twisted  into  a 
U-shaped  body  but  is  secondarily  somewhat  straightened  out,  as 
compared  with  an  ascidian,  since  the  atrium  opens  in  the  opposite 
direction  from  the  mouth.  There  is,  however,  evidence  in  the  posi- 
tion of  the  small,  bent  stomach  and  intestine  that  these  now  free- 
swimming  forms  have  been  derived  from  fixed  ancestors. 

Life  Cycle  of  a  Salpian  (Doliolum). — An  alternation  of  generations 
exists  between  a  sexual  generation  and  a  sexless  generation,  which 
reproduces  only  by  budding.  The  eggs  produced  by  the  sexual  form 
just  described  for  Doliolum  go  through  a  typical  early  development 
and  produce  tadpole  larvae  with  large  body  and  comparatively  small 
tail  (Fig.  30,  A).  The  whole  animal  is  embedded  in  a  thick  coat  of 
jelly.  These  "tadpoles"  undergo  a  metamorphosis  by  resorption  of 
the  tail,  and  produce  a  type  of  individual  much  like  the  sexual  in- 
viduals,  called  nurses  (Fig.  30,  B).  On  the  ventral  side  near  the  heart 
they  have  a  short  finger-like  process  which  produces  primary  buds  by 
a  process  of  constriction.  These  buds  migrate  over  the  surface  of  the 
nurse  by  ameboid  movement  of  the  peripheral  cells,  and  take  up  their 
positions  in  three  longitudinal  rows  on  the  cadophore,  a  dorsal  process 
of  the  body  that  comes  off  near  the  at  rial  end.  The  two  lateral  rows 
are  specialised  as  nutritive  individuals  and  never  leave  the  nurse.  Of 
those  in  the  median  row  some  become  detached  early  and  form  a 
second  generation  of  foster-parents  or  nurses,  while  the  rest  grow  to  be 
sexual  individuals  and  ultimately  separate  one  after  the  other  from  the 
nurse. 

Colonial  Salpians. — The  genus  Salpa  is  the  only  representative 
of  the  Thaliacea  that  is  colonial  in  the  sexual  condition;  chains  of 
individuals  that  remain  as  colonies,  adhering  together  by  means  of 
their  mantle  walls,  are  released  by  the  nurse.  The  development 
from  the  egg  to  the  nurse  is  direct,  without  a  free-swimming  tadpole 
stage.  In  other  respects  they  are  quite  like  the  solitary  salpians. 

ORDER  III.  LARVACEA  (APPENDICULARIANS) 

These  are  free-swimming  pelagic  forms  with  a  much  simplified 
body  region  and  a  persistent  tail  like  that  of  the  larval  ascidian  or 
salpian,  supported  by  a  well-developed  notochord.  The  whole  body 
(Fig.  31)  is  usually  inclosed  in  a  voluminous  gelatinous  envelope  or 
test,  called  a  "  house,"  which  is  secreted  by  the  mantle  ectoderm  just 
as  a  tunicate  secretes  its  wooden  tunic.  The  " house"  is,  however, 


THE  PHYLUM   CHORDATA 


59 


merely  a  temporary  structure  that  serves  for  a  night 's  lodging,  as  it 
were;  for  the  animal  leaves  it  at  intervals  and  is  capable  of  making 
another  " house"  in  an  hour  or  so.  The  tail 
is  rather  loosely  jointed  to  the  body  and 
serves  as  a  flexible  paddle  in  locomotion. 
The  body  is  like  that  of  a  tunicate,  with  the 
whole  food-concentrating  apparatus  reduced 
and  simplified  (Fig.  32).  There  is  only  one 
pair  of  pharyngeal  clefts  that  open  directly 
through  paired  atriopores;  a  condition  sug- 
gestive of  the  larval  atrial  cavities  in  the 
ascidians.  The  endostyle  also  occupies  an 
anterior  position  similar  to  that  of  a  larval 
Amphioxus. 

In  almost  every  respect  the  Larvacea  sug- 


.bd- 


nata 


FIG.  31. — An  individual 
belonging  to  the  class 
Larvacea  (Oikopleurd)  in 
its  gelatinous  "  house." 
Only  the  small  hammer- 
shaped  object  in  the  main 
passage-way  is  the  animal 
itself.  The  arrows  show 
the  current  of  water 
through  the  "house." 
(From  Herdman  after 
Fol.) 


FIG.  32.  —  Diagram  of  a  larvacean  (Appendicularia)  from  the  right  side,  with 
most  of  the  tail  removed,  an,  anus;  ht,  heart;  int,  intestine;  ne,  nerve; 
ne',  candal  portion  of  nerve;  ne.  gn,  principal  nerve  ganglion  ;  ne.  gn  ",  ne. 
gn  '",  first  two  ganglia  of  nerve  of  tail;  noto,  notochord;  oes,  oesophagus;  or. 
ap,  oral  aperture;  oto,  otocyst  (statocyst);  peri,  bd,  peripharyngeal  band; 
ph,  pharynx;  tes,  testis;  stig.  one  of  the  single  pair  of  pharyngeal  clefts,  stigmata; 
stom,  stomach.  (From  Parker  and  Haswell,  after  Herdman.) 


gest  a  permanent  larval  condition  akin  to  neotony  or  psedogenesis. 
The  alternative  view,  that  these  forms  are  prototypic  of  the  ances- 
tral chordate,  is,  we  believe,  not  well  taken;  for  there  are  evidences 
in  the  U-shaped  intestine  that  the  Larvacea  have  come 
sessile  ancestor.  It  is  because  we  consider  the  Ascidiacea  as 


a 


60  VERTEBRATE  ZOOLOGY 

generalized  and  the  Larvacea  as  degenerate  or  paedogenetic,  that 
we  have  arranged  the  orders  of  Urochordata  as  follows: 

Order  I.    Ascidiacea  (Ascidians). 

Sub-Order  1.     Ascidise  Simplices. 
Sub-Order  2.     Ascidise  Composite 
Sub-Order  3.     Ascidiss  Lucise. 

Order  II.    Thaliacea  (Salpians). 

Sub-Order  1.    Cyclomyaria  (e.g.  Doliolum). 
Sub-Order  2.    Hemimyaria  (e.g.  Salpa). 

Order  III.    Larvacea  (Appendicularians). 

SUB-PHYLUM  III.     HEMICHORDATA 

The  status  of  this  group  in  the  chordate  phylum  is  at  best  very 
insecure.  While  the  Enteropneusta,  as  exemplified  by  Balanoglossus, 
seem,  on  account  of  certain  resemblances  to  Amphioxus,  to  have  some 
rather  well-founded  claims  to  vertebrate  relationship,  the  Pterobran- 
chia  and  Phoronidia  are  admitted  to  the  phylum  Chordata  largely  by 
virtue  of  the  almost  unmistakable  affinities  of  Cephalodis€us  with 
Balanoglossus,  and  of  Phoronis  with  Cephalodiscus.  Without  Balan- 
oglossus, it  is  unlikely  that  Cephalodiscus  would  be  considered  as  a 
chordate,  and  without  Cephaldiscus,  there  would  be  little  to  connect 
Phoronis  with  this  phylum. 

ORDER  I.  ENTEROPNEUSTA 

These  are  worm-like,  burrowing  forms  with  numerous  paired  gill- 
slits;  intestine  running  straight  from  mouth  to  terminal  anus.  There 
are  three  main  body  divisions:  anterior  proboscis;  ring-shaped  collar; 
and  segmQuied.tr unk,  resembling  the  body  of  an  annelid  worm.  They 
are  of  moderate  size,  ranging  from  one  inch  to  four  feet  in  length.  The 
burrowing  habits  of  Balanoglossus  (Fig.  33)  reveal  the  significance  of 
these  structural  peculiarities.  Spengel  and  Hitter  have  described 
these  in  some  detail.  The  proboscis  (Fig.  34),  which  is  capable  of  be- 
coming swollen  or  turgid  by  taking  in  water  through  the  collar  pore, 
is  pushed  forcibly  against  the  sand  much  like  the  pig's  snout  in  "root- 
^  V  Sand  is  loosened  and  pushed  aside  until  a  hole  is  made  deep 
1  T-h  for  the  proboscis  to  bury  itself.  Then  the  collar,  by  filling  it- 


THE  PHYLUM   CHORDATA 


61 


self  tightly  with  water  through  the  collar  pore,  comes 
to  fit  itself  rigidly  against  the  sides  of  the  hole  and 
to  act  as  a  fulcrum  for  the  further  operations  of 
the  proboscis.  Thus  progress  is  accelerated.  Sand 
is  quickly  loosened  and  taken  up  by  the  scoop-like 
mouth  which  is  situated  between  the  base  of  the 
proboscis  and  the  ventral  part  of  the  collar.  If 


rob 


cilv 


FIG.  33.— Bd- 
anoglossus.  br, 
branchial  region; 
co,  collar;  gen, 
genital  ridges; 
hep,  hepatic  prom- 
inences formed 
by  hepatic  (liver) 
caeca;  pr,  probos- 
cis. (From  Lull 
after  Spengel.) 


ventv 


FIG.  M.—Balanoglossus.  Diagrammatic  sagittal  section 
of  anterior  end.  card,  s,  cardiac  sac;  div\  diverticulum,  (sup- 
posed notochord);  dors,  n,  dorsal  nerve  strand;  dors,  fin,  dorsal 
sinus;  dors,  ves,  dorsal  vessel;  mo,  mouth;  prob,  proboscis;  prob. 
po,  proboscis  pore;  prob.  skel,  proboscis  skeleton;  vent,  n,  ven- 
tral nerve  strand;  vent,  v,  ventral  vessel.  (From  Parker  and 
Hasvvell,  after  Spengel.) 


we  may  «be  pardoned  a  somewhat  homely  analogy,  we  may  com- 
pare the  collar  of  Balanoglossus  to  a  too-large  collar  of  a  man  and 


62 


VERTEBRATE  ZOOLOGY 


the  base  of  the  proboscis  to  the  too-small  neck  of  the  same  individual. 
The  space  between  the  collar  and  the  neck  in  front  is  like  the  ventral 
mouth  of  Balanoglossus.  Earth  scooped  in  by  this  mouth  is  passed 
rapidly  through  the  alimentary  tract  and  is  deposited  on  the  surface 
by  the  anus.  Digging  in  this  way  the  animal  quickly  disappears 
from  view. 


FIG.  35. — Various  types  of  Enteropneusta,  relatives  of  Balanoglossus.  A. 
Balanoglossus  clavigerus;  B.  Glandiceps  hacksi;  C.  Schizocardium  brasiliense; 
D.  Dolichoglossus  kowalevskii;  a,  anus;  ab,  abdominal  and  caudal  regions;  b, 
branchial  region;  c,  collar;  g,  genital  region;  gp,  gill  pore  or  branchial  cleft;  gr 
genital  wing;  h,  hepatic  region;  m,  position  of  mouth;  p,  proboscis;  t,  trunk.  (From 
Harmer:  A,  B,  and  C,  after  Spengel;  D,  after  Bateson.) 

There  are  about  thirty  species  of  Enteropneusta,  grouped  into  nine 
genera.  Their  mode  of  life  is  essentially  the  same  as  that  of  the  genus 
Balanoglossus.  The  other  genera  differ  mainly  in  the  relative  impor- 
tance and  size  of  the  three  body  regions,  in  the  shape  of  these  regions, 


c.m. 


THE   PHYLUM   CHORDATA  63 

and  in  their  coloration,  which  is  often  quite  striking,  the  prevailing 
colors  being  brilliant  yellows,  red-orange  tints,  and  various  shades  of 
green.  The  most  essential  ana- 
tomical differences  between  the 
genera  are  concerned  with  the 
degree  of  development  of  the 
so-called  notochord. 

A  description  of  the  more 
significant  anatomical  details 
of  Balanoglossus  will  serve 
as  a  characterization  of  the 
group. 

The  Notochord. — The  struc- 
ture which  is  identified  as  homol- 
ogous with  the  true  notochord 
of  typical  chordates  is  identified, 
as  such  largely  on  account  of. 
its  relations  to  other  structures, 
It  consists  of  a  short  thick- 
walled  diverticulum  of  the  mid- 
dorsal  region  of  the  anterior  end 
of  the  alimentary  tract.  The 
diverticulum  projects  forward 
as  a  rod  into  ftie  cavity  of  the 
proboscis  and  is  stiffened  by  a 
Y-shaped  "proboscis  skeleton,"  a 
chitinous  secretion  of  the  sfieath 
of  the  drverticulum.  The  his- 
tological  structure  of  the  noto- 


a. 


36.  —  Schizocardiwn     brasiliense, 


true  notochord  in  that  its  cells 
are  vacuolated.    The  divertic- 


f  ,1       longitudinal,  median  section  through  the 
chord  is  not  unlike  that  of  the  Jj^  en^    6>  blood  space;  &.c?  &.c«, 

b.  c3,  first,  second  and  third  body  cavities; 
cm,  circular  muscles  of  proboscis;  e,  epi- 

•  Tr      £         v  dermis;     lm.     longitudinal     muscles;    m, 

ulum  is  therefore  diagnosed  as  a  mouth!  n>  n;>toch(frd;  nS)  central  ne;vou^ 

notochord  by  virtue  of  its  deri-  system;  d,  dorsal  nerve;  pc,  pericardium; 
vation  from  a  median  dorsal  por-  P«,  proboscis  stalk;  s,  proboscis  skeleton; 
, .  f  ,  -,  v  ,  ,  i  'v,  vermiform  process  or  extension  of  noto- 

tion  of  the  alimentary  tract  and   cnord     (Fron;  Harmer,  after  Spengel.) 
because  it  seems  to  serve  some 

skeletal  function.  Schizocardium  (Fig.  36)  has  a  much  longer  "noto- 
chord" due  to  its  extension  forward  into>  ]ong  vermiform  process. 


64  VERTEBRATE  ZOOLOGY 

The  Pharyngeal  Clefts.— These  structures  take  the  form  of  "  gill- 
sacs,  "  each  of  which  opens  into  the  pharnyx  by  a  U-shaped  slit,  re- 
sembling that  of  Amphioxus,  and  opens  to  the  exterior  by  a  small 
pore.  These  "gill-slit"  openings  through  the  pharynx  are  sup- 
ported by  thin  chitinous  bars,  also  resembling  the  gill-bar  system  of 
Amphioxus. 

The  Neural  Tube. — The  nervous  system  of  Balanoglossus  is  in  gen- 
eral quite  unlike  that  of  the  true  chordate.  Though  it  is  diffuse  and 
rather  poorly  centralized,  a  distinct  ventral  as  well  as  a  dorsal  nerve 
cord  occurs,  the  two  being  connected  at  the  base  of  the  collar  by  a  com- 
missure. In  the  collar  region  a  short  posterior  part  of  the  dorsal  nerve 
cord  seems  to  be  distinctly  tubular.  In  Glossobalanus  and  Ptychodera, 
two  other  enteropneustan  genera,  the  entire  dorsal  nerve  cord  of  the 
collar  is  said  to  be  tubular.  The  claim  therefore  of  the  Enteropneu- 
sta  to  chordate  affinities  are  on  this  score  rather  strong,  for  no  other 
phylum  of  animals  has  a  tubular  dorsal  nervous  system. 

The  most  pronounced  dissimilarities  to  the  Chordates  are  seen  in 
connection  with  the  codomic  cavities,  for  there  are  five  separate  cav- 
ities: an  unpaired  proboscis  cavity,  paired  collar  cavities,  and  paired 
trunk  cavities.  In  this  respect  there  are  rather  striking  suggestions 
of  echinoderm  conditions. 

The  Tornaria  Larva. — Perhaps  the  most  marked  evidences  of  echin- 
oderm affinities,  however,  are  seen  in  the  larva  of  Balanoglossus,  the 
Tornaria  larva  (Fig-.  46)  which  is  so  strikingly  like  that  of  the  young 
Auricularia  larva  of  a  holothurian  that  it  was  originally  classed  as  an 
echinoderm  larva  by  Johannes  Miiller.  The  relationship  that  seems 
to  be  indicated  is  not  so  much  between  the  Enteropneusta  and  the 
V  modern  echinoderms  as  between  a  remote  bilateral  ancestor  of  the 
echinoderms  and  an  equally  remote  pelagic  ancestor  of  the  Enterop- 
neusta. It  is  probable,  however,  that  the  rather  superficial  resem- 
blance between  the  larvae  of  these  two  groups  has  been  overempha- 
sized. Larval  resemblances  present  at  best  a  very  uncertain  basis  for 
phylogenetic  conclusions,  since  so  many  of  the  structures  present  are 
mere  provisional  larval  organs  of  probably  csenogenetic  origin. 

We  must  then  conclude  that  the  Enteropneusta  show  evidences 
of  having  been  derived  from  a  stock  that  was  related  to  the  remotest 
ancestors  of  the  chordates  (the  so-called  protochordates)  and  to-day 
represent  a  rather  unsuccessful  lateral  offshoot  from  the  base  of  the 
chordate  branch  of  the  p\  r]  genetic  tree.  The  closest  linkage  be- 


THE  PHYLUM   CHORDATA 


65 


tween  the  Hemichordata  and  the  vertebrates  is  through  Amphioxus, 
especially  in  the  close  resemblance  between  the  gill-slits  and  gill-bars 
of  the  two  forms. 

ORDER  II.  PTEROBRANCHIA 

These  are  forms  in  which  sessile  life  has  profoundly  modified  the 
primitive  structures.  There  are  two  genera,  Cephalodiscus  and 
Rhabdopleura.  Many  species  of  Cephalodiscus  (Figs.  37  and  38)  are 
found  in  both  deep  and  shallow  water,  mostly  oriental  in  distribution. 
Though  only  about  two  or  three 
millimeters  in  length,  the  individual 
Cephalodiscus  has  certain  unmis- 
takable resemblances  to  Balano- 
glossus.  As  an  adaptation  to  sessile 
life  the  whole  body  is  bent  into  a 
U-shape  so  that  the  mouth  and  anus 
open  close  together.  Instead  of  a 
digging  proboscis,  it  has  a  flattened 
structure  called  the  "buccal  shield'7 
that  overhangs  and  conceals  the 
mouth,  and  whose  cavity  opens  to 
the  exterior  by  paired  proboscis 
pores.  The  collar,  which  has  paired 
collar  cavities  with  pores,  is  pro- 


FIG.  37. — Cephalodiscus  dodeca- 
lophus,  anterior  view.  1,  tentacles; 
2,  proboscis  (buccal  shield);  3,  pig- 


vided  with  from  four  to  six  plume-    ment  band  on  proboscis;  4,  buds; 


fi,  pedicle;  6,  trunk.  (From  Hegner, 
after  Mclntosh.) 


like  tentacles  on  the  dorsal  side. 
These  tentacles  are  provided  with 
ciliated  grooves  which  sweep  food  toward  the  mouth.  There  is  but 
one  pair  of  pharyngeal  clefts  opening  in  the  trunk  just  back  of  the 
collar.  The  notochord  is  a  slender  diverticulum  of  the  proboscis  re- 
gion of  the  dorsal  wall  of  the  alimentary  canal,  practically  identical 
with  that  of  Balanoglossus.  The  nervous  system  is  not  tubular  but 
is  a  mere  plexus  of  nervous  tissue  on  the  dorsal  epidermis  of  the  collar. 
The  trunk  is  short  and  plump,  and  has  paired  body  cavities  in  which 
lie  the  paired  gonads.  The  ecology  and  habits  of  Cephalodiscus  are 
similar  to  those  of  other  colonial  sessile  forms  and  especially  like  those 
of  some  of  the  colonial  ascidians.  They  live  in  comparatively  deep 
water  attached  to  the  bottom,  forming  large  colonies,  each  individual 
of  which  is  embedded  in  a  hollow  pocket  of  the  common  " house." 


66 


VERTEBRATE  ZOOLOGY 


They  depend  for  food  upon  minute  organic  particles  that  come  to  them 
in  the  water  and  may  be  swept  into  the  mouth  by  means  of  the  ciliated 
grooves  of  the  tentacles.  Many  other  sessile  animals  have  a  similar 
feeding  mechanism.  In  fact  there  are  few  fixed  forms  that  do  not 
have  feeding  adaptations  of  this  sort.  Like  the  colonial  ascidians, 


np 


PP 


FIG.  38. — Sectional  view  of  Cephalodiscus.  a,  anus;  bd,  bud  on  stolon  (£)', 
cc,  collar  cavity;  dl,  dorsal  tentacles;  g,  gonad;  i,  intestine;  M,  mouth;  nc,  noto- 
chord;  np,  neural  plate  of  collar  region;  p,  proboscis;  ph,  pharynx;  phc,  pharyngea.1 
cleft  or  gill  slit;  pp,  proboscis  pore;  pc,  proboscis  cavity;  S,  stomach.  (Redrawn 
after  Patten.) 

Cephalodiscus  reproduces   by  asexual  budding  as  well  as  by  eggs 
and  sperm. 

Rhdbdopleura  (Figs.  39  and  40),  a  genus  with  about  four  known 
species  of  minute  tubicolous  forms,  is  obtained  by  deep-sea  dredging. 
They  are  of  microscopic  size,  scarcely  more  than  one-tenth  of  a  mil- 


THE  PHYLUM   CHORDATA 


67 


iimeter  in  length.     Each  individual  inhabits  a  delicate  flexible  tube 
into  which  the  body  may  be  withdrawn.     Rhabdopleura  in  general  re- 
sembles     Cephalodiscus,     but      differs 
chiefly  in  the  lack  of  some  of  the  or- 
gans  possessed   by   the    latter.     There 


FIG.  39.  —  Rhabdopleura. 
a,  mouth;  6,  anus;  c,  stalk; 
d,  proboscis;  e,  intestine;  /,  an- 
terior region  of  trunk;  g,  a  ten- 
tacle. (From  Hegner,  after 
Lankester.) 

are  no  gill-slits  nor  proboscis  pores. 
The  dorsal  region  of  the  collar 
bears  but  a  single  large  pair  of 
feathery  tentacles  which  form  the 
most  conspicuous  part  of  the 
animal. 


st 


ORDER  III.  PHORONIDEA 

Phoronis  is  a  small  tubicolous 
animal  with  a  general  resemblance 
to  Rhabdopleura.  It  has  been 
classified  as  a  gephyrean  (an  aber- 
rant annelid  type) .  The  really  strik- 
ing point  of  contact  is  between  the 
larva  of  Phoronis  and  Balanoglossus.  This  larva  may  be  described 
as  a  somewhat  simplified  Balanoglossus,  since  it  has  the  proboscis, 
the  collar  fringed  with  tentacles,  and  the  short  trunk  terminating  in 


FIG.  40. — Internal  view  of  Rhabdo- 
pleura. a,  anus;  dn,  dorsal  nerve* 
i,  intestine;  m,  mouth;  nc,  notochord; 
p,  proboscis;  pc,  proboscis  cavity; 
ph,  pharynx;  s,  stomach;  si,  stalk; 
t,  tentacles;  tc,  trunk  cavity.  (Re- 
drawn from  Parker  and  Haswell, 
after  Schepotieff.) 


68  VERTEBRATE  ZOOLOGY 

the  anus.  The  "notochord"  is  a  very  short  rudimentary  evagina- 
tion  of  the  alimentary  canal.  There  are  no  pharyngeal  clefts.  In 
Phoronis  we  approach  very  close  to  the  outskirts  of  the  phylum  Anne- 
lida; in  fact  it  is  usually  classed  as  an  aberrant  annelid.  In  a  sense 
the  Phoronidia  may  be  considered  as  a  link  between  the  annelids  and 
the  chordates.  The  propriety  of  classing  them  with  the  chordates  is, 
however,  open  to  serious  question. 

SUB-PHYLUM  IV.    CRANIATA  (THE  VERTEBRATES) 

All  of  the  remaining  chordates  differ  chiefly  from  those  of  the  three 
lower  sub-phyla  in  possessing  a  cranium  of  some  sort  and  a  vastly 
more  complex  brain  than  is  to  be  found  elsewhere.  Although  the  cra- 
niates  are  separated  by  a  wide  gap  from  the  lower  chordates,  they  form 
within  the  group,  especially  when  the  extinct  forms  are  considered, 
a  graded  series  from  the  lowest  to  the  highest  forms,  which  is  taken 
to  indicate  approximately  the  general  course  of  evolution  of  the  sub- 
phylum.  The  group  has  been  compared  to  the  "fairly  reliable  and 
complete  records  of  a  country  during  the  historical  period,"  while 
the  lower  sub-phyla  are  comparable  with  "the  few  scattered  and 
scarcely  decipherable  documents  of  prehistoric  epochs." 

The  characteristics  of  the  Craniata  have  already  been  outlined  in 
the  first  chapter  in  connection  with  the  account  of  vertebrate  mor- 
phology. The  craniates  are  subdivided  into  six  classes,  the  various 
groupings  of  which  are  indicated  in  the  following  table: — 

Div.  1.  ICHTHYOPSIDA — breathing  by  means  of  gills  at  some  period 
in  the  life  history;  therefore  essentially  aquatic  vertebrates. 
They  are  sometimes  called  Anamniota  (Anamnia)  on  ac- 
count of  the  lack  of  an  amnion,  and  Anallantoida  on  account 
of  the  lack  of  an  allantois. 
Class  I.  Cyclostomata — round-mouthed  fishes;  lampreys 

and  hag-fishes. 
Class  II.     Pisces — gnathastome  or  jaw-mouthed    fishes — • 

true  fishes. 
Class  III.    Amphibia — newts,  salamanders,  frogs  and  toads. 

Div.  2.  SAUROPSIDA — breathing  with  lungs  and  never  developing 
functional  gills;  therefore  essentially  terrestrial  vertebrates. 
They  are,  together  with  the  Mammalia,  called  Amniota  be- 


THE  PHYLUM  CHORDATA  69 

cause  they  have  an  amnion,  and  Allantoida  because  they 
have  an  allantois. 

Class  IV.     Reptilia — reptiles,  such  as  lizards,  turtles,  croc* 

odiles  and  snakes. 
Class  V.    Aves — birds. 

Div.  3  AND  CLASS  VI.     MAMMALIA — hairy  quadrupeds  and  other 
mammals. 

The  most  significant  general  subdivision  of  the  Craniata  is 
one  that  recognizes  what  was  probably  the  most  important 
evolutionary  crisis  encountered  by  the  vertebrates:  the  tran- 
sition from  an  aquatic  to  a  terrestrial  mode  of  life.  The  lower 
craniates  (cyclostomes  and  fishes)  are  all  aquatic;  the  Am- 
phibia constitute  a  transitional  group,  but  are  primarily  aquatic, 
in  that  some  of  the  degenerate  forms  are  permanently  aquatic 
and  all  are  essentially  aquatic  during  the  embryonic  and  larval 
periods;  all  of  the  higher  craniates  are  terrestrial  in  the  sense  that 
they  breathe  with  lungs  instead  of  with  gills.  The  Ichthyop- 
sida  are  all  fish-like  creatures  having  the  aquatic  mode  of  locomotion 
and  of  respiration  and  a  circulation  designed  for  gill  respiration.  The 
fish-like  form  common  to  the  members  of  the  group  is  to  be  considered 
as  a  general  adaptation  to  aquatic  life.  They  are  Anamnia  because 
their  eggs  develop  in  water  and  need  no  amniotic  water-bag  to  pro- 
tect the  growing  embryo.  Likewise  they  are  Anallantoida  because 
the  allantois  is  essentially  an  adaptation  for  late  embryonic  respira- 
tion in  the  air  and  is  therefore  not  needed  by  forms  that  develop  gills 
early  in  larval  life. 

The  air-breathing  classes,  Reptilia,  Aves,  and  Mammalia,  are 
Amniota  and  Allantoida  for  the  obvious  reason  that  they  are  terres- 
trial in  embryonic  as  well  as  in  adult  life.  The  grouping  of  the  Rep- 
tilia and  Aves  into  the  Sauropsida  indicates  a  conviction  prevalent 
among  comparative  anatomists  that  there  is  an  unusually  close  affin- 
ity between  these  two  classes.  In  fact  some  go  so  far  as  to  deny  class 
value  to  the  Aves,  on  the  ground  that  they  are  merely  specialized 
flying  reptiles.  The  validity  of  the  subdivision  into  Agnathostomata 
and  Gnathostomata  is  questioned  by  some  authorities  on  the  ground 
that  the  jawless  condition  of  the  cyclostomes  is  said  to  be  due  to  de- 
generation, owing  to  the  highly  specialized  condition  of  the  "rasping 


70  VERTEBRATE  ZOOLOGY 

tongue"  and  its  accessories.  The  claim  is  made  that  some  of  the 
cranial  cartilages  are  the  rudiments  of  a  former  lower  jaw.  If  the  jaw- 
less  condition  is  secondary  there  is  little  reason  for  separating  the 
cyclostomes  from  the  Pisces.  A  more  nearly  acceptable  theory  is  that 
the  cyclostomes  represent  an  early  side  line  of  vertebrate  evolution 
quite  distinct  from  the  true  fishes. 


CHAPTER  III 
THE  ORIGIN  AND  EVOLUTION  OF  THE  VERTEBRATES 

The  problem  of  vertebrate  ancestry  is  an  old  one  and  one  that 
evades  a  direct  solution.  Only  through  circumstantial  evidence  are 
we  at  present  likely  to  reach  even  a  reasonably  satisfactory  answer. 

Certain  postulates  must  be  made,  however,  upon  which  may  be 
built  up  a  theory.  In  the  first  place  it  may  be  assumed  that  the  first 
vertebrates  were  aquatic.  Almost  equally  warranted  is  the  assump- 
tion that  they  were  free-swimming,  active  creatures.  They  were 
also  evidently  axiate  animals  with  fairly  advanced  cephalization,  i.  e., 
with  well- differentiated  heads  and  sense  organs.  They  were  meta- 
meric,  with  well-developed  segmental  musculature  and  with  a  large 
open  coelom.  They  also  had  a  dorsal  tubular  central  nervous  system, 
a  notochord  and  pharyngeal  clefts — three  fundamental  chordate 
characters. 

An  animal  with  these  characters  could  not  be  very  different  from  a 
fish.  Let  us  assume,  then,  with  Osborn,  that  the  first  true  vertebrate 
was  a  "  free-swimming,  quickly  darting  type,  with  double  pointed, 
fusiform  body  in  which  the  segmented  propelling  muscles  are  external 
and  a  stiffening  notochord  is  central."  We  have  found  no  fish  either 
past  or  present  that  appears  to  be  primitive  enough  to  be  considered 
as  the  first  fish.  The  earliest  fossil  fishes  are  armored  types  that  have 
evidently  become  specialized  secondarily  for  bottom  feeding  habits. 
Some  of  the  most  primitive  sharks,  especially  the  Devonian  shark 
Cladoselache  (Fig.  65),  almost  meet  our  expectations  of  what  a  pri- 
mordial fish  may  have  been,  but  even  these  fishes  bear  evidences  of 
having  evolved  from  much  simpler  fish-like  ancestors.  Where  then 
can  we  turn  for  a  true  fish  prototype?  The  existing  lancelets  (Branchi- 
ostoma  or  Amphioxus)  with  their  fusiform  bodies,  notochord,  and 
segmental  musculature,  appear  to  be  the  closest  approximation  we  can 
find  of  what  the  ancestral  fish  creature  may  have  been,  and  one  of  the 
prevailing  theories  of  vertebrate  ancestry  is  the  so-called  "  Amphioxus 
theory,"  a  theory  that  appears  to  the  writer  to  be  more  satisfactory 
than  any  other  and  which  we  shall  herewith  present  in  a  new  form. 

71 


72  VERTEBRATE  ZOOLOGY 

THE  AMPHIOXUS  THEORY  OF  VERTEBRATE  ORIGIN 

Amphioxus,  called  by  someone  the  "chordate  Adam  and  Eve," 
bears  many  evidences  of  being  a  very  antique  type.  Its  cosmopolitan 
distribution,  the  essential  uniformity  of  all  the  different  species,  and 
its  almost  diagrammatic  simplicity  argue  for  its  status  as  a  truly  primi- 
tive creature.  Even  though  it  appears  to  be  somewhat  degenerate  as 
to  its  head  parts  and  though  it  is  suspected  by  some  authors  of  being 
pxdogenetic,  there  is  much  to  show  that  its  fundamental  structures 
are  nearly  prototypic. 

The  habits  of  Amphioxus  show  an  interesting  combination  of  two 
types  of  life,  the  sedentary  and  the  free-swimming.  While  they  are 
capable  of  rapid  and  vigorous  darting  movements  in  the  water,  they 
spend  much  of  the  time  as  sedentary  creatures,  living  in  burrows  in 
the  sand  with  only  the  oral  end  protruding  (Fig.  9) .  This  life  along  the 
sandy  shores  requires  just  exactly  this  double  adaptation,  since  it  is 
in  this  region  that  we  would  find  the  most  effective  tidal  currents. 
The  lancelets  are  evidently  able  to  swim  rapidly  against  the  tidal 
currents  and  to  change  their  locations  as  often  as  the  state  of  the  tide 
demands.  They  also  burrow  into  the  sand  by  vigorous  undulating 
movements  and,  when  once  buried,  they  remain  for  long  periods 
quietly  feeding,  as  shown  in  Fig.  9,  by  means  of  their  unique  food- 
concentrating  mechanism,  a  complex  of  structures  that  is  highly 
adapted  for  sedentary  life  and  ill  adapted  for  active  free-swimming 
life.  The  lancelet  then  has  a  combination  of  two  opposed  tendencies, 
that  of  the  quiet  sedentary  animal,  and  that  of  the  free-living  rapid- 
swimming  animal.  Specialization  might  readily  be  carried  out  in 
either  direction. 

If  specialization  followed  the  lines  of  an  increasing  fixity  of  position, 
we  might  easily  conceive  of  a  type  of  lancelet  that  became  fixed  while 
still  a  larva .  The  functional  stimulus  for  further  development  of  the 
locomotor  musculature  and  the  notchord  would  be  wanting  and  the 
resultant  creature  would  consist  largely  of  a  food-concentrating  and 
digestive  body  without  any  of  the  organs  essential  for  active  life. 
This  we  believe  is  just  what  happened.  Some  of  the  ancestral  lance- 
lets  migrating  into  deeper  waters,  where  the  lack  of  tidal  currents 
would  no  longer  stimulate  the  swimming  about  of  the  larvae  and 
young,  would  remain  stationary  throughout  life,  would  not  even  be 
able  to  make  burrows,  but  would  fix  themselves  to  rocks,  etc.,  at  the 


THE  ORIGIN  AND  EVOLUTION  OF  THE  VERTEBRATES        73 

bottom.  The  secretion  of  a  protective  coat  or  tunic  would  be  a 
logical  consequence  of  such  a  step  and  we  would  have  the  typical 
tunicates.  The  salpians  probably  were  an  offshoot  of  the  primitive 
tunicates  that  migrated  away  from  the  shores,  but  instead  of 
following  the  bottom  they  acquired  the  floating  or  pelagic  habit. 

A  totally  different  situation,  however,  would  meet  those  primitive 
lancelets  that  migrated  from  the  shores  up  the  mouths  of  rivers. 
The  water  currents  would  become  more  constant  and  there  would  be 
less  opportunity  for  sedentary  life .  In  river  currents  the  microscopic 
organic  particles  suspended  in  the  water  would  be  much  less  abundant 
and  hence  it  would  be  more  difficult  to  subsist  by  the  old  lancelet 
method  of  food  concentration.  This  whole  apparatus  as  a  feeding 
apparatus  would  therefore  lose  its  significance  and  would  be  used 
largely  as  a  respiratory  mechanism.  Perhaps  the  first  part  of  the 
mechanism  to  be  lost  would  be  the  one  last  formed  in  ontogeny — -the 
atrium.  Possibly  the  remains  of  the  atrium  would  persist  as  paired 
lateral  ridges,  or  metapleural  folds,  that  might  act  as  lateral  flanges 
or  balancing  organs  in  swimming,  and  may  have  been  the  primordia 
of  the  paired  fins  of  fishes.  New  feeding  methods  had  to  be  acquired 
and  at  least  two  schemes  were  adopted,  one  the  jaw  apparatus  involv- 
ing the  opening  of  a  new  ventral  mouth,  and  the  second  the  oral  fun- 
nel apparatus  with  its  accessory,  the  rasping  tongue.  The  jaw  appa- 
ratus characterizes  the  true  fishes  (Pisces),  while  the  oral  funnel  and 
rasping  "tongue"  apparatus  characterizes  the  Cyclostomata.  Both 
of  these  groups  appear  to  have  originated  at  about  the  same  time  and 
independently  of  each  other.  In  many  ways  the  cyclostomes  have 
been  much  the  less  successful  type  and  comparatively  little  evolution- 
ary progress  has  resulted.  The  true  fishes,  on  the  other  hand,  appear 
to  have  furnished  the  basis  of  all  future  vertebrate  specialization. 
That  the  rapid  stream  environment  was  the  stimulating  factor  in 
the  production  of  the  first  true  vertebrates  has  been  ably  upheld  by 
Prof.  T.  C.  Chamberlin,  on  purely  theoretic  grounds.  He  contends 
that  the  sea  does  not  furnish  the  dynamic  stimuli  necessary  to  bring 
about  the  evolution  of  vertebrate  characters.  The  rivers,  however, 
furnish  just  the  needed  conditions  to  bring  forth  the  complex  of  energy 
elements  that  we  associate  with  a  vertebrate. 

"There  is  only  one  conspicuous  type  that  is  facilely  suited  to 
free  life,  independent  of  the  bottom,  in  swift  streams,  and  that 
is  the  fish-form.  The  form  and  the  motion  of  the  typical  fish  are 


74  VERTEBRATE   ZOOLOGY 

a  close  imitation  of  the  form  and  motion  of  wisps  of  water-grass 
passively  shaped  and  gracefully  waved  by  the  pulsations  of  the 
current.  The  rhythmical  undulations  of  the  lamprey,  which 
perhaps  best  illustrates  the  primitive  vertebrate  form,  and  is  itself 
archaic  in  structure,  are  an  almost  perfect  embodiment  in  the 
active  voice  of  the  passive  undulations  of  ropes  of  river  confervse. 
The  movement  of  the  fish  is  produced  by  alternate  rhythmical 
contractions  of  the  side  muscles,  by  which  the  pressure  of  the 
fish's  body  is  brought  to  bear  in  successive  waves  against  the  water 
of  the  incurved  sections.  In  the  movement  of  a  rope  of  vegeta- 
tion in  the  pulsating  current,  it  is  the  pressure  of  the  pulses  of  the 
water  against  the  sides  of  the  rope  that  give  the  incurvations. 
The  two  phenomena  are  natural  reciprocals  in  the  active  and 
passive  voices. 

"  The  development  in  the  fish  of  a  rhythmical  system  of  motion 
responsive  to  the  rhythm  impressed  upon  it  by  its  persistent 
environment  and  duly  adjusted  to  it  in  pulse  and  force,  is  a 
natural  mode  of  neutralizing  the  current  force  and  securing 
stability  of  position  or  motion  against  the  current,  as  desired. 
Beyond  question  the  form  and  movement  of  the  typical  fish  are 
admirably  adapted  to  motion  in  static  water  and  that  has  been 
thought  a  sufficient  reason  for  the  evolution  of  the  form,  and  so 
possibly  it  may  be,  but  fishes  in  static  water  have  not  as  uniformly 
retained  the  attenuated  spindle-like  form  and  the  extreme  lat- 
eral flexibility  as  have  those  of  running  water.  Among 'these 
latter  it  is  rare  that  any  great  departure  from  typical  lines  and 
from  ample  flexibility  has  taken  place,  while  it  is  common  in  sea 
fishes.  Among  the  latter  not  a  few  have  lost  both  the  typical 
form  and  the  flexibility.  The  porcupine  fish,  the  sea-horse,  the 
flounders,  and  many  others  are  examples  of  such  retrogressive 
evolution,  which  is  doubtless  advantageous  to  them  within 
their  special  spheres  in  quiet  waters,  but  would  quite  unfit  them 
for  life  in  a  swift  stream.  And  if  the  view  be  extended  to  include 
the  low  degenerate  forms,  like  the  Ascidians  (tunicates),  that 
are  by  some  authors  classed  as  chordates,  the  statement  finds 
further  emphasis. 

"It  is  not  difficult  for  the  imagination  to  picture  a  lowly 
aggregate  of  animal  cells,  still  plastic  and  undeterminate  in  or- 
ganization, brought  under  the  influence  of  a  persistent  current 


THE  ORIGIN  AND  EVOLUTION  OF  THE  VERTEBRATES        75 

and  caused  to  develop  into  a  determinate  organization  under  its 
control,  and  hence  to  acquire,  as  its  essential  features,  a  spindle- 
like  form,  a  lateral  flexibility,  and  a  set  of  longitudinal  side  muscles 
adapted  to  rhythmical  contractions,  since  these  are  but  expressions 
of  conformity  and  responsiveness  to  the  shape  and  movement 
normally  impressed  by  the  controlling  environment  upon  plastic 
bodies  immersed  in  it.  The  necessity  for  a  stiffened  axial  tract 
to  resist  the  longitudinal  contractions  of  the  side  muscles  and 
thus  to  prevent  shortening  without  seriously  interfering  with 
lateral  flexibility,  is  obvious,  and  is  supplied  by  the  notochord. 
Thus  by  hypothesis,  the  primitive  chordate  form  may  be  re- 
garded as  a  specific  response  to  the  special  environment  that 
dominated  the  evolution  of  a  previously  indeterminate  ancestral 
form." 

It  will  be  seen  that  the  Amphioxus  theory  of  vertebrate  descent 
carries  with  it  certain  implications: 

(1)  That  the  earliest  pro-fishes  were  of  a  grade  of  organization  not 
very  different  from  Amphioxus. 

(2)  That  the  first  fishes  developed  directly  from  these  pro-fishes 
under  the  influence  of  a  rapid  stream  environment. 

(3)  That  the  ostracoderms,  the  oldest  fossil  fishes  found  in  the  mid- 
dle Ordovician,  are  specialized  bottom-feeding  types  and  are  not  an- 
cestral to  any  of  the  modern  types. 

(4)  That  the  true  ancestral  fishes  were,  according  to  Osborn,  "ac- 
tive, free-swimming,  double-pointed  types  of  fusiform  shape,  adapted 
to  rapid  motion  through  the  water  and  to  predaceous  habits  in  pur- 
suit of  swift-moving  prey." 

(5)  These  primitive  fishes  must  have  existed  before  the  Ordovician, 
i.  e.,  in  Cambrian  times,  which  is  the  earliest  period  of  which  we  have 
positive  fossil  records.    This  makes  the  chordates  as  old  as  any  of  the 
invertebrate  phyla  and  tends  to  detract  from  the  validity  of  any 
theory  involving  the  idea  that  the  chordates  have  been  derived  from 
any  of  the  higher  invertebrate  phyla. 

(6)  That  the  tunicates,  instead  of  being  ancestral  to  Amphioxus, 
have  been  derived  from  an  early  Amphioxus-like  stock. 

OTHER  THEORIES  OF  VERTEBRATE  ANCESTRY 

The  traditional  method  of  phylogenetic  research  has  led  to  the  be- 
lief that  each  higher  group  of  animals  has  been  derived  from  one  of 


76  VERTEBRATE  ZOOLOGY 

the  known  lower  groups.  It  is  generally  believed,  for  example,  that 
the  Arthropoda  are  descended  from  the  Annelida,  the  Annelida  from 
the  Platyhelminthes,  the  Platyhelminthes  from  the  Trochelminthes, 
etc.  It  is  likely  to  be  forgotten  that  these  groups  as  we  know  them, 
both  recent  and  fossil,  are  highly  specialized,  and  that  a  highly  special- 
ized group  is  not  likely  to  retain  sufficient  plasticity  to  give  origin  to 
any  radically  new  departures  that  might  lead  to  new  phyla.  The  mod- 
ern phylogenist  has  come  to  believe  that  the  early  ancestors  of  such 
large  fundamentally  isolated  groups  as  the  phyla  all  originated  far 
back  in  pre-Cambrian  tunes  and  are  therefore  forever  lost  to  us  as 
actual  relics.  Nevertheless  there  are  still  extant  some  theories  that 
derive  the  vertebrates  from  various  contemporaneous  invertebrate 
phyla.  If  the  vertebrates  did  come  off  from  any  known  invertebrate 
types,  what  more  natural  than  to  look  to  the  other  great  metameric 
phyla  for  the  ancestral  conditions?  The  only  truly  metameric  phyla 
among  the  invertebrates  are  the  Annelida  and  the  Arthropoda  and 
there  are  two  rival  theories,  one  claiming  that  the  annelids  and  the 
other  that  the  arthropods  are  the  ancestors  of  the  vertebrates. 

Both  theories  base  their  argument  on  fundamental  morphological 
resemblances  between  the  vertebrate  and  the  annelid,  or  the  arthro- 
pod, as  the  case  may  be.  The  annelid  is  unquestionably  more  general- 
ized in  its  organization  than  the  vertebrate,  and  it  is  true  that  the 
vertebrate  embryo  much  more  closely  resembles  the  annelid  than  does 
the  vertebrate  adult.  There  are  many  striking  homologies  between 
all  three  groups  (annelids,  arthropods,  and  vertebrates)  and  nothing 
could  be  further  from  the  thought  of  the  writer  than  to  deny  that  any 
phylogenetic  relationship  exists  between  them.  Such  a  denial  would 
be  equivalent  to  a  denial  of  the  validity  of  the  whole  principle  of  ho- 
mologies, upon  which  the  science  of  morphology  rests.  An  admission 
of  relationship  between  annelid  and  vertebrate  or  between  arthropod 
and  vertebrate  is,  however,  quite  different  from  an  admission  that  the 
vertebrate  is  descended  from  either  annelid  or  arthropod.  Is  it  not 
much  more  reasonable  to  suppose  that  all  three  of  these  highly  special- 
ized groups,  that  have  so  much  in  common,  have  been  derived  from  a 
primitive  ancestor  characterized  by  the  features  that  all  three  groups 
have  in  common?  Such  an  ancestor  would  be  metameric,  ccelomate, 
with  antero-posterior,  dorso-ventral,  and  bilateral  axes,  with  prob- 
ably ciliary  bands  as  a  mode  of  locomotion,  with  tubular  nephridia, 
with  well-defined  double  nerve-cord  and  paired  ganglia,  and  possi- 


THE  ORIGIN  AND  EVOLUTION  OF  THE  VERTEBRATES   77 

bly  some  sort  of  primitive  paired  appendages.  That  such  an  ances- 
tral form  will  ever  actually  be  discovered  is  highly  improbable,  for 
the  annelids  and  arthropods ,  and  probably  also  the  vertebrates,  were 
old  at  the  dawn  of  Cambrian  times,  and  the  pre-Cambrian  creatures 
are  not  preserved  for  our  edification.  It  would,  however,  be  unfair 
to  leave  the  discussion  of  the  ancestry  of  the  vertebrates  from  annelids 
or  arthropods  without  presenting  some  of  the  evidences  that  have 
been  advanced  in  support  of  these  theories. 

THE  ANNELID  THEORY  OF  VERTEBRATE  ANCESTRY 

The  chief  basis  for  this  hypothesis  lies  in  the  unmistakably  close 
resemblance  between  the  embryonic  characters  of  the  vertebrates  and 
some  of  the  structural  peculiarities  of  the  annelid  worms.  It  has  al- 
ready been  shown  that  the  vertebrate  embryo  is  distinctly  meta- 
meric,  especially  in  the  ccelom  and  its  derivatives.  The  nephridia  of 
the  lower  vertebrates  are  ccelomoducts  with  the  same  relations  as 
those  of  the  annelids.  The  vascular  system,  the  nervous  chain  of 
paired  ganglia,  the  relations  of  the  intestine,  nervous  system,  and 
circulatory  system,  are  so  much  alike  that  a  diagram  (Fig.  41)  of 


St  VERTEBRATE 

FIG.  41.— Reversible  diagram  illustrating  the  Annelid  Theory.  Reversible 
designations  applying  to  both  forms:  S,  brain;  x,  nerve  cord;  H,  alimentary  canal. 
Designations  applying  to  annelid  only:  m,  mouth;  a,  anus.  Designations  applying 
to  vertebrate  only:  st,  stomodseum;  pr,  proctodseum;  nt,  notochord.  (From 
Wilder.) 

these  structures  for  the  annelid  serves,  if  inverted,  as  a  diagram  of  a 
vertebrate.  A  vertebrate  is  said  to  be  merely  an  annelid  turned  over 
on  its  back  and  with  certain  minor  changes  to  adjust  the  animal  to 
the  new  position,  such  as  the  development  of  a  new  mouth  and  a  new 
anus.  Such  positional  reversals  are  not  without  parallel,  for  the  squid 
is  reversed  as  compared  with  other  mollusks,  and  a  king-crab  (Lim- 
ulus)  swims  upside  down.  The  chief  stumbling-block  of  the  anne- 
lid theory  is  the  notochord  of  the  vertebrate;  even  this  is  not  un- 
surmountable  for  there  has  been  found  in  annelids  a  Faserstrang, 
which  is  described  by  Wilder  as  "  a  bundle  of  fibres  running  along  the 


78  VERTEBRATE  ZOOLOGY 

nerve  chain  and  serving  as  a  support.  This  and  the  notochord  lie 
in  precisely  similar  positions  in  relation  to  the  other  organs,  and  in 
both  cases  they  are  inclosed  with  the  nerve  cord  in  a  common  sheath 
of  connective  tissue." 

Coming  back  to  the  idea  that  a  vertebrate  is  an  annelid  turned  over, 
let  us  follow  up  with  the  aid  of  the  diagram  (Fig.  41)  the  results  of  such 
a  reversal.  The  annelid  mouth  is  ventral  and  the  oesophagus  passes 
through  the  brain  in  such  a  way  that  one  pair  of  ganglia  (supracesopha- 
geal  ganglia)  are  dorsal  to  the  oesophagus  while  the  rest  of  the  gan- 
glionic  chain  is  ventral  to  the  alimentary  tract.  The  change  from 
annelid  to  vertebrate  involves  doing  away  with  the  annelid  or  prim- 
itive mouth  and  the  opening  up  of  a  secondary  mouth  on  the  new 
ventral  side,  which  does  not  pass  through  the  nervous  system.  The 
entire  nervous  system,  including  the  supracesophageal  ganglia  (the 
vertebrate  brain)  is  thus  left  on  the  new  dorsal  side.  Two  pieces  of 
embryological  evidence  are  offered  in  support  of  this  contention. 
First,  the  vertebrate  mouth  is  quite  late  in  breaking  through  and  this 
has  been  taken  as  a  sign  that  it  is  an  afterthought.  Second,  there  are 
vestiges  of  the  primitive  or  ancestral  mouth  in  the  neuropore  of  ver- 
tebrate embryos  and  in  the  adult  Amphioxus,  as  well  as  in  the  hypo- 
physis, a  structure  located  where  the  old  mouth  might  have  been,  and 
apparently  without  any  other  significance  unless  it  does  stand  for 
the  point  of  closure  of  the  primitive  mouth.  In  the  annelid  the  blood 
flows  forward  in  the  dorsal  vessel  and  passes  across  through  segmental 
arches  to  a  ventral  vessel  in  which  it  flows  backward.  Reverse  this 
condition  and  we  have  the  primitive  vertebrate  condition  with  the 
blood  flowing  forward  on  the  ventral  side,  crossing  in  certain  special 
arches  (branchial  arches)  to  the  dorsal  side  and  from  there  flowing 
backward.  Some  of  the  annelids  even  have  specialized  branchial  tis- 
sues developed  segmentally  near  the  anterior  end.  In  the  annelid  the 
anus  opens  at  the  posterior  extremity,  but  in  the  vertebrate  a  new 
anus  is  formed  on  the  ventral  side,  leaving  in  the  embryo  a  blind  hind- 
gut,  which  subsequently  disappears,  leaving  a  part  of  the  trunk  back 
of  the  anus  which  has  no  alimentary  tract.  This  is  the  vertebrate 
tail.  Both  the  new  mouth  (stomodseum)  and  the  new  anus  (procto- 
da3um)  are  lined  with  ectoderm. 

" Convincing  as  these  comparisons  seem,"  says  Wilder,  "when 
taken  by  themselves,  the  influence  of  later  investigation  has  tended 
rather  away  from  the  annelid  hypothesis,  and  at  present,  although 


THE  ORIGIN  AND  EVOLUTION  OF  THE  VERTEBRATES        79 

there  are  many  investigators  who  seek  the  ancestor  of  vertebrates  in 
some  worm-like  form,  there  are  few  who  wish  to  definitely  assert 
that  this  ancestor  was  an  annelid." 

The  writer  sees  nothing  in  the  annelid  theory  seriously  out  of  har- 
mony with  the  Amphioxus  theory,  for  Amphioxus  may  have  been 
derived  from  some  early  metameric,  coelomate  type  that  would  be  as 
much  like  an  annelid  as  anything  else.  It  is  much  more  likely,  how- 
ever, that  the  resemblances  between  annelids  and  vertebrates  are 
merely  the  necessary  similarities  that  result  from  the  fact  that  they 
are  constructed  upon  the  same  fundamental  lines.  Given  two  groups 
having  in  common  the  axis  of  polarity,  the  dorso-ventral,  the  bilateral 
axes,  together  with  a  metameric  arrangement  of  organs,  and  the 
expressions  in  structural  differentiation  must  of  necessity  be  fun- 
damentally similar.  Their  differences  are  more  puzzling  than  their 
resemblances,  and  therein  lies  the  real  problem. 

THE  THEORY  OF  ARTHROPOD  ANCESTRY  OF  THE  VERTEBRATES 

This  theory  is  presented  compactly  by  Lull,  as  follows: 

"In  addition  to  the  annelid  theory,  recent  authorities  have 
tried  to  prove  vertebrate  descent  from  the  Arthropoda,  especially 
from  the  more  primitive  arachnoids  such  as  today  are  represented 
by  the  scorpion  and  the  horseshoe  crab  (Limulus)  and  formerly  by 
the  extinct  Merostomata.  By  this  hypothesis  we  must  set  aside 
as  primitive  such  forms  as  Amphioxus  and  the  cyclostomes  and 
start  with  the  highly  specialized  ostracoderms  which  lived  in  Or- 
dovician  and  Devonian  times  and  thus  were  contemporaneous 
with  and  in  general  appearance  and  probable  habits  quite  similar 
to  the  Merostomata.  The  soft  parts  of  the  Merostomata  are  of 
course  unknown,  but  it  is  reasonable  to  suppose  that  they  were 
not  unlike  those  of  the  related  scorpions  and  Limulus,  and,  as 
Patten  has  shown,  especially  in  the  brain  and  cranial  nerves  of 
vertebrates  and  the  fused  cephalothoracic  ganglionic  mass  found 
in  such  arachnoids,  there  are  many  points  of  resemblance  (Figs. 
43  and  44).  Then,  too,  the  sense  organs,  especially  the  eyes,  are 
more  or  less  comparable,  and  there  is  in  Limulus  an  internal  skele- 
tal piece  known  as  the  '  endocranium '  or  sternum  which  serves 
to  protect  the  central  nerve  complex,  and  which  in  general  form 
and  in  its  relation  to  other  parts  resembles  the  primordial 
vertebrate  skull.  Similarities  also  exist  between  the  heart  and 


80  VERTEBRATE  ZOOLOGY 

arterial  systems  of  each  group,  and  the  appendages  may  be  com- 
pared.    There  are,  again,  the  very  arthropod-like  jaws  which' 
Patten  has  demonstrated  for  the  ostracoderm  Bothriolepis,  a 
type  which,  on  the  other  hand,  shows  many  vertebrate-like 
characteristics;  and  the  general  arrangement  of  the  plates  by 


.pa.ey.  Olfactory  Oj>0> 


Coord 

Visual. 
'Masticatory 
Auditory. • 
rLocomot 

Card 
Bra 


FIG.  42. — Diagram  showing  the  supposed  homologies  between:  A,  insect; 
B,  arachnid  (merostome)  and  C,  ostracoderm  (Bothriolepis).  Pro.  C,  procephalon 
or  primitive  head;  Di.  C;  and  Mes.  C,  dicephalon  and  mesocephalon,  usually 
spoken  of  as  thorax;  Met.  C,  metacephalon  or  vagus  region;  Br.  C,  Branchioceph- 
alon,  or  respiratory  region,  d.  end,  ductus  endolymphaticus;  gust,  o,  gustatory 
organ;  I.  ey,  lateral  eye;  pa.  ey,  parietal  eye.  (After  Patten's  "  Evolution  of  the 
Vertebrates  and  their  Kin  "  [P.  Blakiston's  Sons  &  Co.].) 

which  the  cephalothorax  is  covered  is  also  very  similar  in  the 
ostracoderms  and  in  contemporary  arachnoids,  but  unfortu- 
nately for  the  argument  Bothriolepis  is  a  highly  specialized  end- 
form  from  the  Upper  Devonian.  Nevertheless,  while  the  arach- 
noid theory  has  been  set  forth  by  Gaskell  (The  Origin  of  Verte- 
brates, 1908)  and  by  Patten  (The  Evolution  of  the  Vertebrates  and 
their  Kin,  1912)  the  main  thesis  has  received  thus  far  but  little 


•mxl 


Kt2 


FIG.  43. — Sagittal  section  of  a  young  scorpion  (arachnid)  to  be  compared  with 
fig.  44.  c.  a,  carotid  artery;  ch,  cerebral  hemispheres;  e.  t,  parietal  eye  tube; 
ht2-ht3,  heart;  1.  e.  g,  lateral  eye  ganglion;  mxl,  maxillaria;  ra,  mouth;  nch,  noto- 
chord;  oc.  r,  occipital  region;  pae.  g,  parietal  eye  ganglion;  ro,  rostrum;  st.  co, 
stomodaeal  commissure.  (After  Patten's  "  Evolution  of  the  Vertebrates  and  their 
Kin  "  [P.  Blakiston's  Sons  and  Co.].) 


ctt 


pde.  A;     ch.  ol.l. 


FIG.  44. — Sagittal  section  of  a  primitive  vertebrate  embryo,  showing  the  rela- 
tion of  its  principal  organs  to  those  in  the  arachnids  (cf.  fig.  43);  schematic,  a.  np, 
anterior  neuropore;  6.  nv,  belly  navel;  cbl,  cerebellum;  C.  d,  duct  of  Cuvier; 
ch,  cerebral  hemispheres;  cnv,  new  ventral  mouth;  et,  eye  tube;  hy,  hyoid;  I,  lung; 
m,  mouth;  oe,  oesophagus;  ocr,  occipital  region;  oL  I,  olfactory  lobe;  op.  I,  optic 
lobe;  pa..e,  parietal  eye;  pit,  pituitary  body;  pr.mx,  premaxilla;  md,  mandible; 
mx,  maxilla;  S,  stomach;  s.  v,  sinus  venosus;  v,  ventricle.  (From  Patten's  "  Evolu- 
tion of  the  Vertebrates  and  their  Kin  "  [P.  Blakiston's  Sons  and  Co.].) 

81 


82  VERTEBRATE  ZOOLOGY 

recognition;  although  the  evidence,  especially  in  Professor  Pat- 
ten's book,  is  based  upon  an  admirably  executed  piece  of  re- 
search." 

As  Lull  has  intimated,  it  is  highly  improbable  (a)  that  the  ostro- 
coderms,  especially  those  of  the  order  Antiarchi  to  which  Bothriolepis 
belongs,  are  primitive  vertebrates,  and  (b)  that  the  Merostomata  were 
sufficiently  generalized  or  plastic  to  have  given  origin  to  a  group 
radically  different  from  that  to  which  they  belong.  The  theory  re- 
quires the  origin  of  one  highly  specialized  group  of  one  phylum  from  an 
equally  highly  specialized  group  of  another  phylum.  It  is  much  more 
likely  that  the  superficial  resemblances  of  the  two  groups  are  both 
adaptations  for  bottom-feeding,  and  that  the  heavily  armored  condi- 
tion is  evidence  in  both  groups  of  senility.  In  general,  groups  of 
animals  are  thought  of  as  becoming  armored  as  the  result  of 
racial  old  age  and  a  slowing  down  of  the  developmental  vigor  of  the 
constituent  protoplasmic  materials.  The  plastic  young  races  retain 
their  flexibility  and  are  as  a  rule  without  heavy  integumentary  de- 
posits. From  this  point  of  view  the  ostracoderms  fit  very  poorly  into 
the  role  of  primitive  or  ancestral  vertebrates.  If  the  ostracoderms 
cannot  be  used  as  the  link  between  the  fishes  and  the  Merostomata, 
the  whole  fabric  of  vetebrate  phylogeny,  as  erected  by  Patten,  breaks 
up  and  falls  apart.  There  is  no  question,  however,  as  to  the  value  of 
Patten  's  careful  and  exhaustive  studies  of  comparative  anatomy  and 
embryology  of  vertebrates  and  arachnoids,  and  especially  valuable  is 
his  contribution  to  our  knowledge  of  the  anatomy  of  the  ostracoderms, 
a  group  hitherto  all  too  imperfectly  known.  The  chief  fault  that  one 
finds  with  Patten's  method  of  argument  and  exposition  concerns  his 
skillful  illustrations,  which  contain  an  ingenious  intermingling  of 
fact  and  interpretation  that  is  insidiously  convincing  unless  one  be 
on  his  guard.  Figures  42,  43,  and  44  are  among  the  most  character- 
istic of  Patten's  illustrations;  and  they  speak  for  themselves. 

MINOR  THEORIES  OF  VERTEBRATE  ANCESTRY 

There  are  several  other  invertebrate  groups  that  might  conceivably 
have  given  rise  to  the  vertebrates.  One  of  these  is  the  vermian  group 
Nemertea,  a  group  of  uncertain  affinities  but  related  to  the  flat  worms. 
The  Nemertean  Theory  of  the  Origin  of  the  Vertebrates  has  been  advo- 
cated by  Hubrecht,  and  the  argument  is  based  largely  on  the  nervous 


THE  ORIGIN  AND  EVOLUTION  OF  THE  VERTEBRATES    83 

system.  The  nemertean  (Fig.  45)  has  two  lateral  nerve  cords  and  a 
more  slender  dorsal  nerve  cord.  The  three  cords  are  connected  with 
one  another  by  cross  commissures.  It  is  thought  that  in  the  evolution 
of  the  Nemertea  into  vertebrates  the  dorsal  nerve  cord  becomes  the 
central  nervous  system  and  is  then  enlarged  at  the  anterior  end  into 
a  brain;  while  the  two  lateral  nerve  cords  become  compatively  small 


B 


it 


FIG.  45. — To  illustrate  the  Nemertean  Theory  of  the  origin  of  the  Vertebrates. 
A.  A  typical  diagram  of  nemertean.  d,  dorsal  nerve  cord;  gl,  ganglion;  ft,  lateral 
nerve  cord;  v,  intestinal  nerve;  sb,  small  intestinal  branches. 

B.  Typical  diagram  of  vertebrate,  db,  dorsal  brain;  d,  dorsal  nerve  cord;  s, 
sensory,  and  m,  motor  spinal  nerves;  gl,  sympathetic  ganglia;  v,  ramus  intestinalis 
vagi;  sb,  sympathetic  branches.  (From  Wilder,  after  Hubrecht.) 

and  of  secondary  importance,  persisting  however  in  the  so-called 
"  Vagus  system,  rami  lateralis  X"  in  the  lower  vertebrates.  It  can 
scarcely  be  said  that  this  theory  carries  force  in  view  of  the  fact  that 
the  other  structures  of  the  body  show  little  resemblance  to  vertebrate 
conditions. 

Another  theory  of  vertebrate  ancestry  associates  the  latter  with  the 
Echinodermata  through  the  connecting  link  Enteropneusta  (Balan- 
oglossus).  The  Balanoglossus  situation  is  indeed  a  puzzling  one. 
It  does  not  seem  to  fit  in  with  any  of  the  other  theories  of  vertebrate 


84  VERTEBRATE  ZOOLOGY 

ancestry.  The  adult  Balanoglossus,  and  even  more  so  some  of  its 
relatives,  such  as  Harrimania,  shows  certain  striking  chordate  re- 
semblances. The  branchial  orifices  are  similar  in  form  and  relations 
and  remind  one  of  the  pharyngeal  clefts  of  Amphioxus ;  the  notochord 
is  somewhat  doubtful  in  Balanoglossus,  but  in  Harrimania  it  appears  in 
a  much  more  obvious  form  and  has  an  origin  quite  like  that  in  Amphi- 
oxus; the  nervous  system  is  highly  generalized,  consisting  of  four 
longitudinal  cords,  with  the  dorsal  somewhat  more  strongly  developed 
than  the  rest.  In  Glossobalanus  and  Ptychordera,  according  to 
Harmer,  "a  central  canal,  opening  in  front  and  behind,  exists  through- 
out the  entire  length  of  the  central  nervous  system.  .  .  .  Balanoglos- 
sus is  thus  typically  provided  with  a  dorsal,  tubular,  central  nervous 
system,  and  although  it  does  not  extend  beyond  the  limits  of  the 
collar,  it  shows  noteworthy  resemblances  to  Vertebrate  Animals." 

It  is  the  opinion  of  many  students  of  comparative  anatomy  that 
Balanoglossus  is  a  chordate  or  pro-vertebrate  more  primitive  than 
Amphioxus.  According  to  Wilder,  Balanoglossus  and  its  relatives 
the  Enteropneusta  "lie  nearly  in  the  line  of  vertebrate  descent,  and 
represent  an  earlier  stage  than  that  of  the  tunicates.  But  here  the 
chain  seems  to  end,  for  Balanoglossus  is  itself  unusually  isolated  and 
shows  no  close  affinity  to  any  other  invertebrate  types." 

Although  we  may  accept  as  unquestioned  the  above  view  of  the 
chordate  affinities  of  Balanoglossus  there  are  also  very  striking  resem- 
blances between  the  latter  and  certain  echinoderms  which  seem  to 
the  writer  to  be  as  weighty  as  are  those  relating  it  to  the  chordates. 
Foremost  of  these  echinoderm  resemblances  of  Balanoglossus  is  that 
involved  in  the  structure  of  the  Iarva3  of  the  two  groups.  The  Torn- 
aria  larva  of  Balanoglossus  (Fig.  46)  is  compared  with  the  Auricularia 
larva  of  the  holothurian  and  the  Bipinnaria  larva  of  the  starfish.  The 
resemblance  is  obvious.  Moreover,  there  is  in  Balanoglossus  a  system 
quite  like  the  water  vascular  system  of  the  echinoderms,  which  is 
unique  for  that  group.  It  will  be  recalled  that  there  is  a  water-pore 
communicating  with  the  proboscis  coelom  and  a  pair  of  water-pores 
communicating  with  the  paired  collar  cceloms.  "Recent  studies  on 
the  development  of  Echinoderms/'  says  Wilder,  '  have  made  it  prob- 
able that  the  five  body  cavities  of  Balanoglossus  are  represented  in  the 
larvae  of  these  animals;  and  this  materially  strengthens  the  probabil- 
ity of  the  view  that  the  respective  adults  are  also  allied.  It  may  be 
added  that  the  relationship  which  appears  to  be  indicated  is  between- 


THE  ORIGIN  AND  EVOLUTION  OF  THE  VERTEBRATES       85 


Balanoglossus  and  the  bilateral  ancestors  from  which  the  radially- 
symmetrical  Echinoderms  are  probably  descended." 

The  view  is  taken  by  several  authors  that  the  Enteropneusta  and 
the  echinoderms  were  derived  from  a  common  ancestral  stock,  which 
is  now  copied  in  a  simplified  form  by  the  larvae  of  both  groups.  "  The 
common  characters  of  all  the  larvae  are,"  says  Wilder,  "  bilaterality, 


FIG.  46. — Comparison  of  Tornaria  larva  with  larval  echinoderms.  Main 
ciliated  bands  in  black,  lesser  systems  cross-lined.  Upper  row  ventral  aspect; 
lower  row  right  lateral  aspect.  A.  A  ',  Tornaria;  B.  B  ',  Auricularia  (sea  cucum- 
ber); C.  C  ',  Bipennaria  (star-fish).  (From  Lull,  after  Wilder.) 


transparency,  locomotion  by  bands  of  cilia,  and  pelagic life."  A  com- 
mon ancestor  having  these  characters  may  well  haye  existed.  "The 
lineal  descendents  of  this  hypothetical  ancestor  chose  two  paths,  the 
one  leading  to  the  Echinodermata,  the  other  to  Balanoglossus,  the 
Tunicata,  Amphioxus,  and  eventually  the  Vertebrate."  This  view 
makes  the  Enteropneusta  ancestral  to  the  tunicates  and  the  tunicates 
ancestral  to  Amphioxus.  In  a  previous  discussion  of  the  Amphioxus 
theory  we  have  dealt  with  the  tunicates  as  degenerate  derivatives  of 
Amphioxus-like  ancestors;  just  the  reverse  of  the  opinion  expressed 
by  Wilder.  It  should  also  be  said  that,  although  Amphioxus  is  ob- 
viously not  at  the  very  bottom  of  the  vertebrate  ancestral  trunk, 


86  VERTEBRATE  ZOOLOGY 

there  is  little  evidence  of  its  having  descended  from  any  form  at  all 
like  Balanoglossus.  Rather  would  we  believe  that  Balanoglossus  is 
an  early  lateral  offshoot  from  the  main  line  of  chordate  stock  that 
leads  more  directly  to  Amphioxus  and  the  vertebrates. 

SUMMARY  OF  THE  THEORIES  OF  VERTEBRATE  ANCESTRY 

After  a  review  of  these  various  and  contradictory  theories  as  to  the 
origin  of  the  vertebrates,  are  we  any  nearer  a  solution  of  this  por- 
plexing  problem  than  when  we  started?  Certainly  it  cannot  be 
claimed  that  the  problem  is  solved,  but  at  least  we  have  examined  the 
question  and  have  considered  the  various  possibilities.  Of  all  the 
alternatives,  the  one  that  makes  Amphioxus  a  central  main-line  an- 
cestral form,  perhaps  slightly  degenerate,  but  only  a  little  specialized, 
seems  best  supported  by  evidence.  Amphioxus,  on  the  one  hand,  is  so 
much  like  the  vertebrates  that  it  has  been  classed  as  an  acraniate 
vertebrate  by  so  able  a  writer  as  Osborn.  It  is  also,  on  the  other  hand, 
unquestionably  related  to  the  tunicates.  The  assumption  of  an  Am- 
phioxus-like  creature  as  a  common  ancestor  of  vertebrate  and  tunicate, 
and  the  diagnosis  of  the  Hemichordata  as  an  early  lateral  offshoot  of 
a  still  more  primitive  chordate  trunk,  seems  much  more  logical  than 
alternative  assumptions.  It  also  rids  us  of  the  necessity  of  deriving 
one  phylum  from  the  highly  differentiated  members  of  another  phy- 
lum, and  is  therefore  more  acceptable  than  annelid,  arthropod,  or 
nemertean  theories. 


CHAPTER  IV 
CLASS  I.   CYCLOSTOMATA 

The  name  "  round-mouth  eels"  is  often  applied  to  these  fish  -like 
forms  to  distinguish  them   from  the  "jaw-mouth"  fishes 


tomes).  The  group  consists  of  two  distinct  types,  the  "  hag-fishes" 
and  the  lampreys.  Both  of  these  have  a  superficial  resemblance-to  the 
eels,  but  they  differ  from  the  group  to  which  the  eels  belong  (Pisces)  / 
in  many  important  respects.  Some  of  the  differences  are  to  be  V 
interpreted  as  evidences  of  primitiveness  and  others  of  specialization 
and  degeneration.  Since  there  are  no  certain  fossil  remains  of  the  cy- 
clostomes,  it  is  an  exceedingly  difficult  matter  to  decide  as  to  which 
of  their  characters  are  palingenetic  (truly  primitive)  and  which  are 
csenogenetic  (due  to  specialization  or  degeneration)  .  Evidently,  how- 
ever, the  characters  that  are  of  universal  occurrence  in  the  group  are 
more  likely  to  be  primitive  than  are  those  in  which  the  "hags"  dif- 
fer from  the  lampreys. 

The  cyclostomes  are  a  minor  class  of  vertebrates  as  compared  with 
the  other  five  classes,  since  there  are  only  a  few  genera  and  species. 
They  are  also  of  secondary  significance  phylogenetically;  for  they  are 
believed  to  represent  a  comparatively  unsuccessful  lateral  branch  of 
the  vertebrate  ancestral  tree,  that  came  off  probably  from  an  Amphi- 
oxus-like  stock  prior  to  the  origin  of  the  fishes  proper.  If  this  view  is 
valid,  the  true  fishes  and  all  of  the  other  vertebrate  classes  repre- 
sent an  evolutionary  series  totally  independent  fron^the  cyclostomes. 
Any  attempts,  therefore,  to  establish  detailed  homologies  between  the 
two  groups  must  be  viewed  with  suspicion.  On  the  whole,  the  cyclo-  , 
stomes  have  departed  Jess  .widely  from  the  Amphioxus-like  prototype  ** 
than  have  the  fishes,  and  in  that  sense  they  represent  a  lower  grade 
of  vertebrate  organization. 

The  characters  in  which  the  cyclostomes  in  general  differ  from  the 
fishes  are  as  follows: 

External  features: 

A)  No  jaws.  Attempts  have  been  made  to  homologize  the  so-called 
"tongue  cartilages"  with  the  first  visceral  or  mandibular  arch, 
but  the  comparison  is  far-fetched. 

87 


88  VERTEBRATE  ZOOLOGY 

2.  The  mouth  is  round  and  is  closed  only  b}'  the  end  of  the  "  tongue." 
faj There  is  a  median  unpaired  nostril,  and  the  nasal  passage  in 
some  (hag-fishes)  opens  into  the  mouth;  in  others  (lampreys) 
it  ends  blindly  beneath  the  brain. 

4.  The  branchial  clefts  are  in  the  form  of  pouches  or  pockets,  a  char- 

acter that  has  given  to  the  group  the  name  "  Marsipobranchii." 
The  number  of  clefts  is,  in  general,  greater  than  in  fishes. 

5.  There  are  no  paired  appendages. 

6.  There  are  no  scales^ 

7.  The  lateral  line  organs  lie  in  open  grooves. 
Internal  features: 

1.  There  is,  in  lieu  of  a  masticatory  apparatus,  a  complex  rasping 
apparatus,  called  a  "tongue",  that  is  armed  at  the  mouth  end  with 
chitinous  teeth,  is  supported  by  several  cartilages,  and  is  worked  by 
a  specialized  set  of  muscles.  Since  both  hags  and  lampreys  have  this 
apparatus,  in  spite  of  their  very  different  modes  of  life,  it  is  likely 
that  this  is  a  very  old  character  and  represents  an  evolutionary  ex- 
periment even  more  ancient  than  does  the  jaw  apparatus  of  fishes. 

fy  The  notochord  is  a  persistent  unbroken  rod  much  like  that  of 
Amphioxus,  but  extends  only  to  the  hind-brain  and  is  covered  with 
an  extra  sheath  of  connective  tissue. 

£S\  The  vertebra  are  represented  by  primitive  neural  arches^quite 
separate  from  the  notochord. 

4.  The  brain  is  small  but  typically  vertebrate  in  structure,  with 
the  vagus  nerve  not  included  in  the  cranial  region. 

(5)  The  cranium  is  entirely  beneath  the  brain  and  fqrms  neither 
sides  nor  roof  for  the  latter.  Attempts  have  been  made  to  homologize 
the  cartilages  of  this  cranium  with  those  of  the  embryonic  fish 
ill. 

The  gonads  give  off  their  products  into  the  ccelom.  Eggs  and 
sperm  make  their  exit  directly  to  the  exterior  through  genital  pores 
situated  near  the  urinary  opening. 

7.  The  inner  ear  is  more  primitive  than  in  fishes,  having  only  one 
or  two  semicircular  canals. 

8.  The  afferent  branchial  arteries  go  directly  to  the  gill  pouches  in- 
stead of  into  the  arch  between  two  gill  clefts. 

9.  A  branchial  basket  composed  of  cartilaginous  bars  and  rods  forms 
a  support  to  the  branchial  part  of  the  pharynx.     This  cannot  suc- 
cessfully be  compared  with  the  branchial  arches  of  fishes. 


CYCLOSTOMATA 


89 


The  my  atomic  musculature,  the  median  fins,  the  digestive,  circulatory, 
and  excretory  system  are  much  as  in  the  fishes. 

A  more  special  account  of  the  characters  of  cyclostomes  will  best  be 
presented  when  the  two  sub-classes  Myxinoidea  and  Petromyzontia 
are  compared. 

SUB-CLASS  I.  MYXINOIDEA 

These  "hag-fishes"  or  "borers"  are  called  myxinoids  because  of 
their  habit  of  producing,  when  captured,  great  quantities  of  slimy 


-da 


D 

FIG.  47. — Myxine.  A.  External  view  of  entire  animal;  the  rows  of  pores  are 
openings  of  mucous  glands;  no  eyes.  B.  Ventral  view  of  anterior  end,  showing 
terminal  nostril,  oral  hood  with  buccal  tentacles;  ba,  single  pair  of  branchial  aper- 
tures (<?/.  D);  rhg,  openings  of  mucous  glands.,  C/ inner, ear  showing  single  semi- 
circular canal.  D.  In-ternal  anatomy,  ac,  auditory  capsule;  6s,  branchial  sacs, 
opening  by  means  of  a  common  branchial  tube  (cbt)  into  the  common  branchial 
aperture  (ba)',  g,  gut;  h,  heart;  nc,  notochord;  nt,  nasal  tube;  sc,  spinal  chord; 
tc,  tongue  cartilages;  tm,  tongue  musculature.  (Redrawn  after  Parker  and  Haswell.) 

mucous  jelly.  It  is  said  that  one  large  specimen  will  make  a  bucket- 
full  of  jelly.  Hags  lead  a  quasi-parasitic  life,  in  that  they  commonly 
enter  the  gills  or  mouths  of  dead  or  disabled  fishes  and  remain  inside 
till  they  have  so  completely  gutted  the  quasi-host  that  only  the  shell 


90  VERTEBRATE  ZOOLOGY 

remains.  It  seems  unlikely  that  they  attack  living  active  prey,  as  do 
the  lampreys,  but  they  undoubtedly  do  prey  upon  fishes  caught  in 
gill-nets  or  upon  trot-lines.  On  account  of  these  habits  the  hags  have 
acquired  a  well-earned  unpopularity  among  the  fishermen  of  the 
North  Sea  and  elsewhere.  The  flesh  of  the  prey  is  shredded  by  means 
of  the  strong  rasping  apparatus,  and  the  digestive  tract  is  so  capa- 
cious that  one  good  meal  lasts  some  time.  All  active  hunting  is  done 
at  night,  for  the  hags  are  blind;  the  day  is  spent  buried  in  the  mud  of 
the  sea  bottom  at  depths  around  three  hundred  fathoms,  where  they 
are  themselves  safe  from  enemies.  They  swim  swiftly  with  an  eel-like, 
undulatory  motion.  When  caught  they  secrete  from  the  skin  glands  a 
great  quantity  of  gelatinous  mucus.  Hags  are  said  to  be  the  only  verte- 
brates that  are  specifically  hermaphroditic.  This  character  may  be 
associated  with  their  solitary  life  and  with  the  fact  that  they  are  blind. 
Some  of  the  anatomical  characters  of  the  myxinoids  that  differ 
from  those  of  the  petromyzonts  are  herewith  listed.  These  charac- 
ters are  illustrated  in  Figs.  47  and  48. 

I.  The  mouth  is  terminal  and  there  is  no  real  buccal  funnel. 

2:  The  naso-pituitary  sac  (nasal  passage  and  infundibulum)  opens 
into  the  pharynx  and  through  it  water  is  drawn  into  the  gills  while 
the  mouth  is  engaged  in  feeding. 

3.  There  are  four  pairs  of  tentacles  surrounding  the  mouth  and  the 
terminal  nasal  opening,  which  have  been  compared  with  the  oral 
tentacles  of  Amphioxus. 

4.  The  branchial  skeleton  (basket)  is  poorly  developed,  since  it 
does  not  have  to  withstand  strong  suction. 

5.  Dorsal  arcualia  (vertebral  arches)  are  confined  to  the  tail  region 
or  extend  only  slightly  forward  from  the  tail. 

6.  No  spiral  valve  in  the  intestine. 

7.  There  is  a  row  of  mucous  sacs  on  each  side. 

8.  The  brain  has  no  distinct  cerebrum  or  cerebellum. 

9.  The  eyes  are  degenerate  and  without  muscles  or  nerves. 

10.  There  is  no  regional  specialization  of  the  median  fin. 

II.  There  is  only  one  semicircular  canal  in  the  inner  ear. 

12.  The  tongue  apparatus  is  larger  and  more  elaborate  than  in  the 
lampreys  and  this  pushes  the  gill-slits  further  back. 

13.  Some  of  the  myxinoids  have  a  larger  number  of  gill-slits  than 
the  lampreys,  which  is  considered  a  primitive  character. 

14.  The  pronephros  (larval  kidney)  functions  in  the  adult. 


CYCLOSTOMATA 


91 


15.  The  eggs  are  large  and  rich  in  yolk.  It  naturally  follows  that 
the  cleavage  is  meroblastic  and  the  development  is  direct  without 
larval  metamorphosis. 

It  is  probable  that  numbers  3,  4,  5,  6,  8,  10,  11,  13,  and  14  represent 
more  primitive  conditions  than  those  found  in  the  petromyzonts; 


mg 


be 


D 

FIG.  48. — Bdellosloma.  A.  External  view  of  whole  animal,  showing:  be,  bran- 
chial clefts;  mg,  mucous  glands.  B.  group  of  eggs  adhering  by  anchor-like  hooks. 
C.  ventral  view  of  anterior  end,  showing  somewhat  ventral  nostril,  ventral  mouth, 
and  oral  tentacles;  be,  branchial  clefts;  x,  resophageo-cutaneous  duct.  D.  Larva, 
showing  functional  eye.  (Redrawn  mainly  after  Dean.) 

while  numbers  1,  2,  7,  9,  12,  and  15  represent  csenogenetic  characters 
resulting  from  specialization  or  degeneration. 


SUB-CLASS  II.  PETROMYZONTIA 

^ 

The  lampreys  live  in  both  fresh  and  salt  water,  the  marine  species 
being  larger  than  those  inhabiting  streams.  They  live  an  active 
predaceous  life,  attacking  frequently  fishes  much  larger  than  them- 
selves, such  as  sharks.  Their  mode  of  attack  is  to  give  chase  and  to 
attach  themselves  to  the  body  of  the  prey  by  means  of  the  sucker- 


92  VERTEBRATE  ZOOLOGY 

mouth  or  oral  funnel,  which  is  lined  with  chitinous  teeth  to  help  in 
securing  a  firm  hold.  The  flesh  of  the  fish  is  then  lacerated  by  the 
rasping  tongue  and  swallowed.  Ultimately,  of  course,  the  prey  is 
killed  and  then  feeding  may  be  carried  on  at  leisure.  While  they  are 
feeding,  the  mouth  is  closed  and  respiration  is  carried  on  by  incurrent 
and  excurrent  streams  of  water  through  the  branchial  clefts.  The 
branchial  part  of  the  pharynx  is  cut  off  from  the  alimentary  part  and 
ends  blindly,  so  that  respiration  is  independent  of  the  mouth. 

The  fresh-water  lampreys  are  lovers  of  rapid  waters  and  are  often 
seen  in  rocky  streams  attached  by  the  mouth  to  stones. 

The  Petromyzontia  represent  an  evolutionary  stage  a  step  or  two 
beyond  that  occupied  by  the  Myxinoidea  and  a  formal  list  of  their 
characters  furnishes  an  interesting  comparison  with  those  given 
above  for  the  latter.  The  characters  of  the  adult  lamprey  are  shown 
in  Fig.  49. 

.  1.  The  mouth  is  provided  with  a  perfect  oral  funnel,  used  as  a 
vacuum  cup,  and  armed  with  chitinous  teeth. 

^  2.  The  naso-pituitary  sac  ends  blindly  beneath  the  brain. 

,3.  N6  buccal  tentacles. 

.4.  Branchial  skeleton  a  stiff  intricate  basket  of  cartilaginous  bands 
and  rods. 

.5.  Vertebral  arches  extend  well  forward  from  the  tail  and  are  of  a 
more  advanced  structure  than  in  the  myxinoids. 

•  6.  A  rudimentary  spiral  valve  is  present  in  the  intestine,  probably 
indicating  a  specialization  of  the  intestine  paralleling  that  seen  in  the 
sharks  and  certain  other  fishes. 
7.  No  distinct  mucous  sacs. 

«  8.  The  cerebral  hemispheres  are  distinct  and  a  band-like  cerebellum 
is  recognizable. 

9.  Eyes  well  developed,  with  good  muscles  and  nerves. 

J.O.  The  median  fin  system  is  specialized  into  two  dorsal  fins  and 
a  caudal  fin. 

f  11.  There  are  two  semicircular  canals  in  the  inner  ear,  a  condition 
intermediate  between  that  seen  in  the  myxinoids  and  that  in  the 
true  fishes,  where  three  canals  are  always  present. 
12.  " Tongue"  apparatus  less  elaborate. 

•13.  Number  of  gill-slits  uniformly  7,  more  primitive  than  fishes,  but 
more  specialized  than  in  some  of  the  myxinoids. 

•  14.  Pronephros  non-functional  in  the  adult. 


B 


\l i \. 


D 


Ol 


C 


1 


FIG.  49. — Petromyzon.  A.  Extarnal  view  of  entire  animal  from  specimen;  na, 
median  nasal  aperture.  B.  Ventral  view  of  head  showing  funnel-like  oral  sucker 
armed  with  chitinous  teeth;  end  of  "tongue"  in  mouth  opening.  C.  Dorsal 
view  of  head,  showing  median  nostril.  D.  Internal  anatomy:  6s,  branchial  sac 
cut  open  to  show  gill  tissue;  6,  brain;  da,  dorsal  aorta;  h,  heart;  N,  notochord; 
na,  nasal  aperture;  leading  into  naso-pituitary  sac  (nps),  ns,  nasal  sac;  oe,  cesoph- 
agus  quite  independent  of  pharyngeal  diverticulum  (p};  of,  oral  funnel;  sc,  spinal 
cord;  tc,  tongue  cartilage.  E.  Ventral  view  of  cranium:  ac,  auditory  capsule; 
oc,  olfactory  capsule.  F.  Dorsal  view  of  cranium:  N,  notochord.  G.  Dorsal 
view  of  brain:  ch,  cerebral  hemispheres;  cb,  cerebellum;  d,  diencephalon;  mo, 
medulla  oblongata;  ol.  olfactory  lobe;  opl,  optic  lobe;  pn,  pineal  body.  (Re- 
drawn after  Parker  and  Haswell.) 

93 


94 


VERTEBRATE  ZOOLOGY 


15.  The  eggs  are  comparatively  small  and  with  little  yolk.  Cleav- 
age is  holoblastic.  The  larva  has  a  long  life  and  undergoes  a  radical 
metamorphosis  into  the  adult  state. 

Thus  the  lampreys  may  be  considered  as  a  step  in  advance  of  the 
hags  in  numbers  1,  2,  3,  4,  5,  6,  8,  10,  11,  13,  14.  In  other  respects 


FIG.  50. — Spawning  of  the  Brook-Lamprey  (Petromyzon  wilderi).  On  the  right 
of  the  figure  a  male  is  attached  to  the  head  of  a  female.  (From  Cambridge  Nat. 
Hist.,  after  Dean  and  Summer.) 

they  show  either  more  primitive  conditions  or  merely  different  ad- 
aptations for  their  totally  different  life. 

J  Life  Cycle  of  the  Lamprey.  The  lamprey  breeds  in  fresh  water,  even 
in  the  case  of  the  marine  forms.  The  male  coils  the  tail  about  the 
body  of  the  female  in  the  spawning  act  (Fig.  50)  and,during  a  vigorous 


CYCLOSTOMATA 


vibration  of  the  two  bodies,  eggs  and 
sperm  are   extruded   in   close  contact. 
The  eggs  sink  to  the  gravelly  bottom 
in  a  place  that  has  been  cleaned  up  for  >^s> 
a  "nest."     The  nest  has  been  prepared  ' 
by  moving  many  stones,   both   males 
and  females  using  the  buccal  funnel  for 
this  purpose. 

The  egg,  which   measures   about   a 
millimeter  in    diameter,    goes    rapidly  < 
/through  cleavage,   blastula,    and    gas- 
trata   stages   and  forms   a  tiny   larva 
/which  has  come  to  be  known  as  "  Am- 
mocoetes,"  because  when  first  discovered 
it  was  believed  to  represent  a  separate 
genus  of  lowly  chordates. 

The  most  significant  characteristics 
of  the  Ammoccetes  larva  (Fig.  51)  are  5 
thtfse  in  which  it  strikingly  resembles  ^ 
/Aniphioxus:  (1)  a  hood-like  upper  lip 
resembling  the  oral  hood  ofAniphioxtis ; 
(2)  a  well-defined  parietal  or  median 
eye ;  (3)  a  food-concentrating  apparatus 
consisting  of  an  endostyle  and  dorsal 
mucous  groove;  this  implies  a  similar 
mode  of  feeding;  (4)  median  fins  con- 
tinuous and  unspecialized. 

Ammoccetes  is  more  advanced  than 
Amphioxus   in    other  respects,   as   for 
example : — the  paired  eves  that  lie  dee 
in  the  head,  a.  smp.11  nrajnjmm,  a  muc 


•1. 


ft  ,'- 
*' 

"& 


FIG.    5!.— Ammocoetes     larva 
of  Petromyzon,  enlarged  sagittal 


section,    bd,  bile  duct;  e#,  ciliated 


more  advanced  brain,  a  reduced  num- 

E  1         —       _ ot/v-'iAiA-'i.A.      i/u/j   (LPJ.J.C;  VALIV./LJ.  oj/.   v^inct IA>VJ. 

_er_pt   branchial^clefts,  a  concentrated  groove;  da,  dorsal  aorta  ;•/«,  cav- 

kidriey  (pronephros)  /adistinct  ventral  %  of  brain  J*  gills;-*,  intestine; 


notochord;  n^  neural    tube;   oe-, 

oesophagus;  ^  oral  papillae;  pf,  pericardium  with  heart  removed;  p»,  pineal 
eye;  p»^  pronephros  showing  nephric  funnels;  s^,  spiral  valve;  ifc,  thyroid  forming 
fr«.)m  endostyle;  ^  velum;  no,  ventral  aorta;  ««,  intestinal  vein.  (From  Lan- 
J*jster's  "  Treatise  on  Zoology/'  Vol.  IX,  Goodrich.) 


96  VERTEBRATE  ZOOLOGY 

After  living  in  the  larval  state  for  from  three  to  four  years,  during 
a  few  weeks  in  the  winter  it  undergoes  a  profound  metamorphosis  of 
structure  and  habits  and  emerges  as  a  small  juvenile  lamprey. 
During  the  metamorphosis  the  buccal  funnel  is  developed;  the  eyes 
come  to  the  surface  and  begin  to  function;  the  median  fin  becomes 
specialized;  the  skull  and  branchial  basket  become  more  elaborate, 
the  gall  bladder  is  lost  except  for  a  few  vestiges.  The  most  remarkable 
transformation,  however,  concerns  the  pharyngeal  region  and  the 
food-  concentrating  mechanism.  The  dorsaLpart.  which  represents 
gr™^y°j  ^ornrnes  pinched  off  from  the  old  pharynx 


+.hg.i.  hrpfllrg  into  the  rnouth.     The  endo- 


becomes  pinched  off  frr>m  the  ventral  floor  of  the  pharynx 
and  coiled  up  into  a  thyroid  aland.  The  middle  part  of  the  pharynx 
remains  as  the  blind,  lung-like  branchial  sac  which  operates  independ- 
ently of  the  mouth. 

All  of  these  facts  seem  to  point  unmistakably  to  an  affinity  between 
the  cyclostomes  and  the  cephalochordates.  This  helps  to  bridge  the' 
gap  between  the  vertebrates  and  the  lower  chordates,  It  has  been 
suggested  that  Amphioxus  is  simply  a  case  of  psedogenesis,  or  ex- 
treme racial  senescence  involving  de*XTk>piiiunt  arrrestecTin  a  larval 
condition.  —  According  to  tnis  view,  which  is  not  acceptable  to  biolo- 
gists^ln  general,  Amphioxus  is  a  permanent  larva  of  some  species  of 
cyclostome.  The  fact  that  Ammocoetes  spends  as  much  as  four  years 
in  the  larval  stage  lends  color  to  this  contention,  for  it  indicates  a 
developmental  retardation,  that,  if  carried  a  step  farther,  might  result 
in  a  permanent  paedogenetic  condition  akin  to  that  which  occurs  in 
the  Amphibia  (Axolotl  larva  of  Amblystoma).  There  are,  however, 
arguments  against  this  view  which  would  be  out  of  place  here.  We 
have  already  put  ourselves  on  record  as  supporting  the  theory  that 
the  ancestor  of  the  vertebrates  was  Amphioxus-like.  Possibly,  how- 
ever, such  an  ancestor  was  more  like  Ammoccetes  than  like  Amphi- 
oxus. 


1 


CHAPTER  V 
CLASS  II.    PISCES  (TRUE  FISHES) 

The  fishes  are  at  present,  and  have  been  since  Silurian  and  Devo- 
nian times,  the  dominant  creatures  of  the  waters,  both  fresh  and  salt. 
The  chronological  history  of  the  fishes  is  well  shown  in  Fig.  52.  Al- 
though the  environment  has  been  practically  Constant  both  as  to 
temperature  and  chemical  constitution  the  evolution  of  form  and 
function  has  gone  on  rapidly.  "This  indicates,"  says  Osborn,  "that 
a  changing  physicochemical  environment,  although  important,  is  not 
an  essential  cause  of  the  evolution  of  form." 

Although  the  aquatic  environment  may  in  a  sense  be  thought  of  as 
constant  it  is  not  a  uniform  or  homogeneous  medium,  for,  within 
aquatic  confines,  there  are  several  life  zones  that  differ  radically 
from  one  another.  There  are:  (a)  the  region  of  river  or  tidal  currents 
in  which  the  fish  must  be  active,  swift-moving,  and  predaceous;  (b)  the 
surface  strata  of  still  bodies  of  water,  where  life  may  be  comparatively 
passive  and  where  only  a  moderate  speed  is  necessary;  (c)  the  region 
at  the  bottom,  which  may  be  either  at  moderate  depths  or  abysmal, 
where  life  may.  be  sluggish.  These  types  and  derivatives  from  them 
are  concisely  shown  in  the  accompanying  pictorial  table  (Fig.  53). 

The  body  form  of  the  type  living  in  currents  and  depending  for 
food  and  safety  on  swiftness  is  naturally  the  double-pointed,  elon- 
gated, submarine-shaped  animal  (Fig.  53,  a  to  c),  illustrated  by 
some  sharks,  the  pickerel,  the  trout,  the  salmon,  and  many  of  the  min- 
nows. Few  of  these  swift-moving  fishes  have  heavy  armor,  nor  have 
they  any  excessive  development  of  fins,  spines,  or  other  projecting 
structures  that  might  interfere  with  swift  progress  through  the 
water.  The  dog-shark  or  common  spiny  dog-fish  (Fig.  62)  may  be 
taken  as  an  excellent  example  of  this  type  of  .fish,  a  type  that  is  be- 
lieved to  copy,  perhaps  more  nearly  than  any  other,  the  ancestral 
fish  form. 

The  fishes  of  the  open  seas  or  lakes  living  at  moderate  depths  or 
near  the  surface,  where  they  are  little  affected  by  currents,  are  often 
of  the  deep-bodied,  laterally  compressed  type  (Fig.  53  h,  i).  A  good 

t? 


98 


VERTEBRATE  ZOOLOGY 


example  of  this  group  is  the  sun-fish,  representing  that  vast  assem- 
blage of  modern  fishes,  the  Acanthopterygii,  which  includes  more 
species  than  all  other  groups  of  fishes  combined.  Extreme  cases  of  the 
laterally  compressed  type  are  seen  in  the  "Head-Fishes"  and  in  Zan- 
clus  (Fig.  86),  which  are  about  as  high  as  they  are  long  and  are  very 
much  compressed.  This  foreshortening  of  the  long  axis  accompanied 


FIG.  52. — Origin  and  Adaptive  Radiation  of  the  Fishes.  Dotted  areas  rep- 
resent groups  still  existing;  black  areas  represent  extinct  groups.  (After  Osborn 
and  Gregory.) 

by  corresponding  increase  in  height  may  be  taken  as  a  symptom  of 
racial  senescence;  for,  according  to  this  view,  there  has  been  a  retarda- 
tion in  growth  vigor  down  the  principal  axis  accompanied  by  a  marked 
acceleration  of  growth  in  the  secondary  (dorso-ventral)  axis. 

The  bottom-feeding  type  (Fig.  53,  j,  k,  I)  is  one  that  involves  many 
grades  and  types  of  specialization.  In  general,  bottom  feeders  are 
depressed  dorso-ventrally  and  are  broad  bilaterally.  Common  rep- 
resentatives of  this  adaptive  assemblage  are  the  skates  and  rays,  the 
extinct  ostracoderms,  and  several  types  of  bottom-feeding  teleosts. 
They  are  essentially  the  oldest  or  most  senescent  of  the  fish  types, 
as  evidenced  by  the  comparative  suppression  of  the  primary  or  longi- 
tudinal axis  and  the  secondary  or  dorso-ventral  axis  by  the  tertiary 
or  bilateral  axis. 

Thus  it  will  be  seen  that  the  fishes,  although  in  a  less  varied  habitat 


PISCES 


99 


than  that  of  terrestrial  animals,  tend  to  radiate  adaptively  in  several 
main  and  many  minor  directions.    This  has  occurred  not  once  in  the 


FIG.  53. — The  principal  types  of  body  form  in  fishes,  a,  6,  swift  movin^com- 
pressed,  fusiform;  c,  d,  e,  elongated,  swift,  fusiform  types,  grading  into—/,  g,  elong- 
ated eel-like  forms;  h,  i,  laterally  compressed,  slow  moving,  deep-bodied;  j,  k,  I,  lat- 
erally depressed,  flat,  bottom-feeding.  (After  Osborn's  "  Origin  and  Evolution  of 
Life."  [Charles  Scribner's  Sons].) 

evolution  of  fishes,  but  many  times.  Every  large  group  of  fishes  ex- 
hibits " adaptive  radiation"  in  Osborn's  sense;  for  we  find  in  nearly 
every  order  of  fishes  the  swift  fusiform  types,  the  short  laterally  com- 


100  VERTEBRATE  ZOOLOGY 

pressed  types,  the  broad,  shallow,  bottom-feeding  types,  and  many 
minor  types. 

STRUCTURAL  FEATURES  OF  THE  FISHES 

The  fish  in  its  typical  form  is  essentially  an  aquatic  mechanism — a 
submarine  automaton.  Like  a  submarine  vessel  it  has  a  fusiform 
shape;  steering  and  balancing  devices;  hydrostatic  appliances,  such 
as  air  reservoirs;  a  mechanism  for  extracting  oxygen  from  the  water; 
optical  devices  adapted  for  aquatic  vision;  and  instruments  for  de- 
tecting vibrations  in  water,  warning  of  the  approach  of  the  enemy  or 
of  the  nearness  of  prey. 

The  following  formal  characterization  of  the  Class  Pisces  will  serve 
to  distinguish  them  from  other  classes: 

1.  Jaws:  fishes  proper  are  all  Gnathostomata  (hinged-mouthed 

as  distinguished  from  the  Cyclostomata  or  round-mouthed 
eels. 

2.  Gills  or  Branchiae. — Their  method  of  respiration  is  distinctly 

aquatic  throughout  life,  though  accessory  organs  for  air- 
breathing  occur  in  several  distinct  orders  of  fishes.  The  gills 
are  vascular  processes  of  the  walls  of  the  branchial  clefts. 

3.  The  Circulation  is  built  about  the  gill  system.     The  blood  is 

pumped  forward  from  the  ventral  heart  through  the  gills  and 
is  thence,  as  arterial  blood,  carried  backward  in  the  dorsal 
aorta.  This  scheme  of  circulation  wherever  found  will  be 
interpreted  as  primarily  aquatic. 

4.  The  Heart  is  a  single  S-shaped  muscular  tube,  with  but  one 

auricle,  one  ventricle,  and  a  bulbus  arteriosus,  and  receives 
only  venous  blood. 

5.  Tail. — The  principal  organ  of  locomotion  is  the  tail,  terminated 

by  a  paddle-like  expansion,  the  caudal  fin,  and  sculled  by 
means  of  the  powerful  segmental  muscles. 

6.  Fins. — These  are  of  two  sorts,  the  paired  and  the  median  fins. 

The  paired  fins,  pectoral  in  front  and  pelvic  behind,  are  homol- 
ogous to  the  fore  and  hind  limbs  of  terrestrial  vertebrates 
and  are  supported  by  bony  or  cartilaginous  rays  articulated 
with  a  simple  pectoral  or  pelvic  girdle,  which  may  be  either 
bony  or  cartilaginous.  These  appendages  are  essentially 
balancing  organs  though  they  may  be  modified  for  various 
purposes,  or  even  lost.  The  median  fins  occur  in  both  ven- 


PISCES  101 

tral  and  dorsal  positions  and  may  be  greatly  modified  or 
wanting  in  places.  They  are  supported  by  bony  or  car- 
tilaginous rays. 

-  7.  Exoskeleton. — The  integumentary  units  are  scales  of  various 
sorts,  and  are  found  in  every  degree  of  exaggeration  or  degen- 
ation.  In  general  they  may  be  said  to  be  placoid,  ganoid,  cy- 
cloid, or  ctenoid  in  form  and  structure.  Sometimes  the  scales 
fuse  together  to  form  a  coherent  armor. 

8.  Endoskeleton. — The   endoskeleton    consists   of   appendicular 

and  axial  elements.  The  appendicular  skeleton  is  mentioned 
under  "Fins."  The  axial  skeleton  consists  of  a  bony  or 
cartilaginous  cranium  and  a  vertebral  column  more  or  less 
completely  organized,  consisting  of  bone  or  cartilage.  A 
notochord  persists  in  some  of  the  more  primitive  orders. 

9.  Lateral  line  of  sensory  organs. — These  strictly  aquatic  sense 

organs  are  found  arranged  in  linear  tracts  along  the  sides  and 
over  the  head.  Their  exact  function  is  not  definitely  known, 
but  they  are  probably  associated  with  the  perception  of  vi- 
brations in  the  water. 

10.  Olfactory  organs. — These  are  paired-and  end  blindly,  not  com- 

municating with  the  pharnyx  as  in  terrestrial  animals  and 
in  the  hag-fishes. 

11.  Auditory  Organs. — These  are  entirely  internal  and  have  no 

communication  with  the  exterior.  They  serve  largely  the 
function  of  equilibration,  though  they  also  perceive  vibra- 
tions. 

12.  Eyes.-^The  eyes  are  much  like  those  of  other  vertebrates,  ex- 

cept that  they  are  lidless,  and  have  spherical  lenses  for  short- 
range  vision  in  .the  water. 

13.  Brain. — The  brain  is  small  and  shows  no  flexures.    It  neverthe- 

less has  all  of  the  characteristic  features  of  the  vertebrate 
brain,  though  there  are  but  ten  cranial  nerves. 

14.  Spinal  Cord:  like  that  in  other  vertebrates. 

15.  Alimentary  Tract. — The  pharynx  is  extensive  and  perforated 

by  branchial  clefts.  The  oesophagus  is  simple  and  short,  for 
in  the  fish  there  is  no  neck.  The  stomach  is  little  differenti- 
ated. The  intestine  is  short  and,  in  order  to  increase  the  di- 
gestive surface,  a  spiral  valve  is  often  present,  especially  in 
all  of  the  more  primitive  orders. 


102  VERTEBRATE  ZOOLOGY 

16.  A  Swim-Bladder  occurs  in  all  but  the  Elasmobranchii,  the 

Holocephali,  and  in  a  few  degenerate  teleosts.  It  is  a  gas- 
filled  bladder,  derived  from,  and  frequently  connected  with, 
the  pharynx.  In  some  fishes  it  is  used  as  an  accessory  lung, 
but  it  is  usually  for  hydrostatic  purposes. 

17.  Kidneys. — The  nephridial  system  consists  of  elongated  bodies 

situated  in  the  median  dorsal  part  of  the  ccelom.  The  units 
of  the  system  are  nephric  tubules  that  have  nephrostomes, 
funnel-like  openings  into  the  coelom.  The  functional  kidney 
is  a  mesonephros. 

18.  Gonads. — The  ovaries  and  testes  are  simple  sac-like  structures 

that  have  ducts,  oviducts  and  vasa  deferentia,  developed  in 
connection  with  the  primitive  nephridial  ducts,  as  in  other 
groups. 

19.  Eggs. — The  eggs  of  different  fishes  range  from  large,  heavily- 

yolked  eggs  with  chitinous  shells,  as  in  the  modern  elasmo- 
branchs,  to  small  pelagic  eggs  of  many  modern  teleosts. 
The  eggs  are  for  the  most  part  fertilized  in  the  open  water,  but 
many  fishes  of  various  orders  practice  internal  impregnation 
and  are  viviparous. 

An  understanding  of  the  majority  of  these  characters  will  doubt- 
less be  acquired  in  connection  with  the  laboratory  exercises  that  ac- 
company courses  in  vertebrate  zoology,  but  further  comment  on  three 
of  the  most  significant  characteristics  of  fishes,  the  fins,  the  respira- 
tory organs,  and  the  integument,  seems  to  be  necessary. 

THE  FINS  OF  FISHES 

Of  all  characters  of  fishes  the  fins  are,  perhaps,  the  most  distinctive 
since  they  are  adaptations  for  aquatic  life.  Analogous  structures  have 
been  secondarily  developed  by  reptiles  and  mammals,  such  as  the  ex- 
tinct ichthyosaurs  and  the  porpoise.  (Fig.  4). 

The  median  fin  system  appears  primitively  as  a  continuous  median 
fold  supported  by  cartilaginous  or  bony  rays,  running  from  just  back 
of  the  head  on  the  dorsal  side,  round  the  tail  and  ending  behind  the 
vent  on  the  ventral  side.  According  to  one  view  the  median  fin  sys- 
tem bears  no  relation  to  the  paired  fin  system,  which  is  thought  of  as 
being  derived  independently  from  gill  septa.  The  prevailing  view 
or  "  continuous  fin-fold  theory,"  however,  holds  that  originally  the 
median  fin  system,  instead  of  terminating  back  of  the  vent,  bifur- 


PISCES 


103 


cated  around  the  latter  and  continued  forward  as  two  lateral  fin-folds 
that  reached  almost  to  the  head.  Two  pieces  of  evidence  support 
this  theory.  The  first  is,  that  in  Amphioxus  the  median  fin-fold 
actually  does  fork  and  continue  forward  on  the  ventral  side  as  the 
metapleural  folds.  The  second  is  that  in  the  primitive  shark  Clado- 
selache  (Fig.  65)  the  paired  fins  have  rays  parallel  to  one  another  much 
as  in  the  median  fin  system,  and  give  the  impression  that  they  are 
merely  parts  of  a  system  continuous  with  the  latter.  Two  diagrams 
are  here  presented  that  illustrate  the  origin  of  the  paired  fins  from 


BrF 


FIG.  54. — Fin-fold  origin  of  paired  fins.  A.  The  hypothetical  undift'erentiated 
condition.  B.  The  manner  in  which  it  is  thought  that  the  permanent  fins  were 
derived  from  the  continuous  fin-folds.  AF,  Anal  fin;  An,  anus;  BF,  pelvic  fins; 
BrF,  pectoral  fins;  D,  continuous  dorsal  fin-fold;  FF,  posterior  dorsal  fin;  S  and 
S,  paired  ventral  lateral  fin-folds;  Sl,  median  ventral  fin-fold  continuous  with 
S  and  S;  SF,  superior  or  dorsal  lobe  of  caudal  fin.  (From  Wiedersheim.) 

lateral  fin-folds.  The  first  (Fig.  54)  is  that  of  Wiedersheim,  and  the 
second  is  adapted  from  that  of  Kingsley  (Fig.  55),  involving  double 
dorsal  as  well  as  double  ventral  fin-folds. 

The  most  primitive  types  of  fishes  or  fish-like  creatures  have  the 
median  fin  system  unbroken  and  regionally  unspecialized,  but  most 
of  the  fishes  proper  have  the  continuous  median  fin  subdivided  into 
one,  two  or  three  dorsal  fins,  a  caudal  fin,  and  one  or  two  anal  or  ven- 
tral fins.  The  degree  of  development  of  these  various  regional  special- 
izations of  the  median  fin  system  varies  greatly  according  to  the  habit 
and  degree  of  specialization  of  the  group.  In  the  more  generalized 
types  the  size  and  degree  of  elaboration  of  these  fins  remain  well 
within  the  bounds  of  bodily  symmetry,  but  in  some  of  the  highly 


104 


VERTEBRATE  ZOOLOGY 


specialized  groups  these  fins  may  become  greatly  exaggerated  as  in 
Zanclus  (Fig.  86)  and  in  Dendrochirus  (Fig.  90). 

The  most  significant  evolutionary  processes  concern  the  caudal  fin 
and  in  this  region  of  the  fish's  body  we  have  changes  that  parallel 
those  that  have  occurred  in  the  caudal  regions  of  various  land  animals, 
involving,  on  the  one  hand,  more  or  less  pronounced  foreshortening 
or  atrophy,  and,  on  the  other  hand,  excessive  prolongation  or  hyper- 
trophy of  the  tail  region.  Beginning  with  what  appears  to  be  the 


FIG.  55. — Diagram  of  the  origin  of  the  median  and  paired  appendages  from 
lateral  fin-folds.  The  arrows  indicate  the  points  of  junction,  dorsal  and  ventral, 
of  the  paired  fin-folds  with  the  median  fin-folds.  (Modified  after  Kingsley.) 

ancestral  condition,  we  have  a  type  of  caudal  fin  called  diphycercal 
(Fig.  56,  A),  which  is  evenly  developed  both  above  and  below  the 
notochord  of  the  tail.  The  supporting  rays  above  the  notochord 
(epichordal  rays)  are  as  well  developed  as  those  below  the  notochord 
(hypochordal  rays).  A  type  of  caudal  fin  that  appears  to  be  next  in 
order  of  antiquity  is  the  heterocercal  type  (Fig.  56,  B),  in  which  the 
epichordal  rays,  and  consequently  the  upper  lobe  of  the  fin,  are 
much  less  strongly  developed  than  the  hypochordals.  This  type  of 
fin  is  found  in  most  elasmobranchs,  in  the  Holocephali,  in  the 
Chondrostei  and,  in  a  modified  form,  in  the  Holostei.  A  third  type, 


PISCES 


105 


or  homocercal  caudal  fin  (Fig.  56,  E),  is  the  result  of  a  foreshortening 
of  the  terminal  portion  of  the  caudal  axis  and  results  in  the  typically 
" fish-tailed"  type  of  fin.  In  a  teleost  fish  such  as  the  salmon  the 
young  fish  has  first  a  diphycercal,  then  a  heterocercal,  and  finally  a 
homocercal  type  of  fin.  Various  modifications  of  these  three  main 
types  are  found  and  will  be  commented  upon  in  appropriate  places. 


FIG.  56. — Types  of  Caudal  Fins.  A,  Diphycercal,  with  equal  dorsal  and  ven- 
tral lobes;  B,  Heterocercal  (Selachii);  C,  Modified  diphycercal  (some  teleosts); 
D,  Heterocercal  (Chondrostei) ;  E,  Homocercal  (teleosts);  F,  Abbreviated  het- 
erocercal (some  Holostei).  of,  anal  fin;  axl,  axillary  process;  cr,  caudal  fin  rays; 
def,  dorsal  lobe  of  caudal  fin;  df,  dorsal  fin;  ef,  epi caudal  lobe  of  caudal  fin;  ha, 
haemal  arches;  hf,  hypocaudal  lobe  of  caudal  fin;  na,  neural  arches;  nt,  notochord; 
r,  dermal  fin  rays.  (From  Lankester's  "Treatise  on  Zoology,"  Vol.  IX.  [A  &  C. 
Black].) 

The  most  primitive  type  of  paired  fin  is  believed  to  be  that  seen  in 
Cladoselache  (Fig.  65).  The  " lobe-fins"  of  the  Crossopterygii  are 
next  in  primitiveness,  while  theTms~bf  other  groups  are  specialized 
types  derived  from  these  two  primitive  types.  As  will  appear  later 
the  " lobe-fin"  is  the  most  nearly  hand-like  in  architecture  and  is 
believed  to  have  given  rise  to  the  hand-type  of  paired  appendage  seen 
in  primitive  land  vertebrates. 


106 


VERTEBRATE  ZOOLOGY 


eT.o 


THE  RESPIRATORY  ORGANS  OF  FISHES 

The  characteristic  respiratory  organs  of  aquatic  vertebrates  are 
gills  or  branchiae.  These  structures  are  finely  divided  outgrowths  of 
the  ectodermal  or  endodermal  epithelium  lining  the  branchial  clefts. 
The  number  of  clefts  or  gill-slits  varies  from  five  to  seven  in  number, 
each  cleft  being  separated  from  its  neighbors  by  branchial  septa. 
The  most  primitive  fishes  have  the  larger  number  of  branchial  clefts 
and  the  more  modern  types  have  regularly  five.  Heptanchus,  some- 
times mentioned  as  the  most  primitive  living  species  of  shark,  has 
seven  clefts,  Hexanchus,  another  primitive  shark,  has  six,  while  the 
elasmobranchs  in  general  have  five  fully  developed  clefts  and  a  vestig- 
ial anterior  first  cleft  called  a  spiracle.  The  spiracle  or  rudimentary 
first  cleft  is  also  found  among  the  most  primitive  Teleostomi  (Cros- 

sopterygii  and  Chondrostei) ,  and  is  present 
in  the  embryos  of  Teleostei  and  Holostei, 
but  is  closed  before  hatching.  In  the 
Holocephali,  an  aberrant  group  of  elasmo- 
branch  fishes,  the  fifth  cleft  is  closed  in 
the  adult,  thus  reducing  the  number  of 
functional  clefts  to  four.  It  will  be  re- 
called that  the  pro-vertebrates  have  much 
larger  numbers  of  pharyngeal  clefts,  over 
fifty  pairs  in  Amphioxus  and  even  larger 
numbers  in  some  tunicates.  The  cyclos- 
tomes  have  on  the  whole  larger  numbers 
of  clefts  than  the  true  fishes.  Though  the 
hag-fishes  of  the  family  Myxinidce  have  no 
more  than  six  pairs,  those  of  the  family 
Bdellostomidce  have  as  many  as  fourteen 
pairs,  while  the  lampreys  all  have  seven 

.    ...    .  pairs.    The  direction  of  evolution  appears 

FIG.  57.— External  gills  in  *  /** 

embryo  torpedo,    d,  cloaca;  to  be  one  of  reduction  in  number  of  clefts 

ex.  b,  external  gills;  el.  o,  elec-  from  around  fifty  in  the  Amphioxus-like 

trie  organ;   ys,    yolk  stalk.  ancestor,  fourteen  to  six  in  the  cyclostomes, 
(From     Bridge,      Cambridge  '  .  ' 

Nat.  Hist.,  Vol.  VII.)  seven  to  five  in  the  true  fishes,  and  four  in 

the  Holocephali. 

The  openings  of  the  clefts  to  the  exterior  differ  in  different  groups 
of  fishes.  Among  the  elasmobranchs  the  usual  situation  is  that  each 


cx.b 


PISCES 


107 


cleft  opens  separately  and  is  not  covered  by  any  flap  or  operculum; 
though  in  Chlameidoselachus  the  primitive  " frilled  shark"  (Fig.  67,  A) 
each  cleft  has  a  backwardly  directed  flap  or  gill-cover.  In  the  Holo- 
cephali  the  first  three  clefts  are  covered  by  an  operculum  and  only 
the  fourth,  or  last  functional  cleft,  opens  freely  to  the  outside.  In  the 
great  majority  of  Teleostomi  and  in  the  Dipneusti  the  five  clefts  are 
covered  with  a  flap-like  operculum,  capable  of  opening  and  closing 
and  effectively  protecting  the  branchial  filaments  from  injury.  In 
some  of  the  eels  and  in  other  specialized  types  of  teleosts  the  gills  are 


FIG.  58. — Diagram  of  gills  of  fishes.  A,  Horizontal  section  through  the  head  of 
an  Elasmobranch;  B,  Similar  section  of  a  Teleost.  be,  branchial  cavity;  bl, 
branchial  lamellae;  c,  coelom;  eba,  external  branchial  aperture;  hy.  a,  hyoid  arch; 
hy.  c,  hyo-branchial  cleft;  h,  interbranchial  septum;  r),  nasal  organ;  oes,  oesoph- 
agus; op,  operculum;  pq,  palatoquadrate  cartilage;  Ph,  pharynx;  sp,  spiracle; 
s.  ps,  spiracular  pseudobranch;  1-5,  1st  to  5th  branchial  arches.  (From  Bridge, 
after  Boas.) 

completely  covered  with  a  fold  of  skin  and  the  only  exit  is  through 
one  or  a  pair  of  small  water-pores. 

Two  quite  different  and  distinct  kinds  of  gills  are  found  among 
fishes:  external  and  internal  gills. 

External  gills  are  purely  larval  or  ^mbryonic  organs  and  are  not 
functional  in  any  adult  fish;  though  their  homologues  are  found  in 
the  perennibranchiate  Amphibia,  believed  to  be  psedogenetic  or  per- 
manent larval  types.  External  gills  are  finely  branched  processes  of  the 


108 


VERTEBRATE  ZOOLOGY 


ectodermal  epithelium  of  the  branchial  clefts.  They  are  found  in  the 
embryos  of  many  elasmobranchs  (Fig.  57)  and  in  some  teleosts.  A 
notable  case  of  larval  gills  is  seen  in  the  advanced  larva  of  Polyp- 
terus  (Fig.  70,  C). 

Internal  gills  are  true  functional  gills  of  adult  fishes.  They  are  finely 
divided  diverticula  of  the  endodermal  epithelium  of  the  branchial 
clefts.  Their  location  is  well  shown  in  the  diagrams  of  elasmobranch 

and  teleost  heads  (Fig.  58). 

THE    AIR-BLADDER    AND   ACCES- 
SORY ORGANS  OF  RESPIRATION 

In  all  of  the  groups  of  fishes 
above  the  elasmobranchs  there  is 
a  single  or  paired  air-bladder,  a 
sac-like  diverticulum  of  the  phar- 
ynx derived  from  either  dorsal  or 
ventral  sides  of  the  alimentary 
tract.  It  is  in  all  cases  supplied 
with  blood  from  the  "pulmonary 
artery"  and,  primitively  at  least, 
subserves  two  functions:  that  of 
a  hydrostatic  or  buoyancy  organ 
and  that  of  an  accessory  respira- 
tory organ  or  primitive  lung.  In 


FIG.  59. — -Respiratory  labyrinth  of 
the  Climbing  Perch  (Anabas  scandens) 
exposed  by  removal  of  part  of  operculum. 
ba  ',  first  branchial  arch;  to,  labyrinth- 
iform  organ;  op,  operculum.  sbc,  supra- 
branchial  cavity.  (From  Bridge). 


FIG.  60. — Accessory  respiratory  organs  of  the  cat-fish,  Clarias,  as  seen  after 
removal  of  operculum.  a,  anterior  arborescent  organ;  b.  a  1-4,  first  four  branchial 
arches;  d.  b.  c,  dorsal  extension  of  left  branchial  cavity;/,  modified  gill-filaments; 
op,  base  of  operculum;  p,  posterior  arborescent  organ.  (From  Bridge.) 


PISCES  109 

the  most  primitive  teleostome  fishes,  the  Crossopterygii,  it  is  used  as 
a  lung  when  the  water  is  foul;  in  A mia  it  is  constantly  functional 
as  an  air-breathing  apparatus;  while  in  the  Dipneusti  (lung-fishes)  it 
is  an  elaborately  pouched  lung,  used  to  tide  the  fish  over  a  period  of 
drought. 

In  certain  other  fishes  that  have  acquired  terrestrial  habits,  such 
as  the  Climbing  Perch,  Anabas  (Fig.  59),  and  in  the  air-breathing  eel, 
Clarias  (Fig.  60),  there  is  an  extensive  post-branchial  chamber  pro- 
vided with  labyrinthine  or  arborescent  elaborations  of  the  epithelium 
that  are  highly  vascular  and  play  a  pulmonary  role. 

THE  INTEGUMENT  OF  FISHES 

The  integument  of  fishes  differs  from  that  of  land  vertebrates  in 
being  soft  and  slimy  on  the  surface,  though  usually  well  protected 
beneath  the  soft  epidermis  by  means  of  scales  or  plates  composed  of 
hard  materials  such  as  bone  and  ganoin.  The  slimy  condition  of  the 
epidermis  is  not  due  to  a  deposit  from  the  water,  as  is  commonly 
believed,  but  is  a  mucous  secretion  from  numerous  cutaneous 
glands. 

The  scales  of  fishes  differ  materially  from  those  of  reptiles,  birds, 
and  mammals  in  that  they  are  composed  exclusively  of  dermal  ele- 
ments. In  the  land  vertebrates  the  scales  are  mainly  cornifications 
of  the  epidermis.  The  great  majority  of  fishes  are  more  or  less  com- 
pletely covered  with  scales  of  moderate  size,  but  some  fishes  such  as 
the  eels,  cat-fishes,  etc.,  have  secondarily  lost  their  scales  or  have 
them  in  a  degenerate  condition,  imbedded  deeply  in  the  skin;  other 
fishes  have  the  scales  replaced  by  bony  plates,  which  may  form  a  more 
or  less  solid  armor,  as  in  the  trunk  fishes  (Fig.  93)  and  the  porcupine 
fishes  in  which  the  plates  are  provided  with  spines.  The  possession 
of  a  coating  of  small,  equally  distributed  scales  is  considered  as  the 
primitive  or  generalized  condition  for  fishes,  and  the  loss  of  scales 
or  the  development  of  heavy  plates,  as  specialized  or  senescent  condi- 
tions. 

Four  main  types  of  scales  are  distinguished:  placoid,  ganoid,  cy- 
cloid and  ctenoid.  The  placoid  scale,  characteristic  of  elasmobranch 
fishes,  is  believed  to  be  the  original  type  of  scale.  It  consists  of  a 
basal  plate  of  bony  substance  derived  from  the  dermis  and  a  spine- 
like  external  protuberance  covered  with  an  enamel-like  substance  de- 
rived from  the  epidermis.  In  the  sharks  and  their  kin  it  is  clear  that 


110  VERTEBRATE  ZOOLOGY 

from  this  type  of  scale  are  developed  the  true  teeth — the  latter  being 
merely  enlarged  and  flattened  placoid  scales  derived  from  the  oral  in- 
tegument. Thus  the  placoid  scale  is  the  ancestor  of  all  true  verte- 
brate teeth. 

The  ganoid  scale  is  an  archaic  type  of  integumentary  unit,  found 
only  in  the  so-called  ganoid  orders  of  Teleostomi  and  in  a  few  prim- 
itive teleosts.  The  ganoid  scale  is  believed  to  be  derived  from  the 
placoid  condition  by  the  loss  of  the  spike-like  protuberance  and  by 
the  addition  of  a  hard  external  coating  of  glistening  substance  called 
ganoin  which  is  secreted  by  the  dermis  and  is  not  homologous  with 
enamel.  Usually  ganoid  scales  are  rhombic  in  form  and  are  laid 
like  tiles,  but  in  some  ganoid  fishes  the  scales  overlap  like  shingles. 
Cycloid  and  ctenoid  scales  are  of  similar  structure  to  ganoid  scales 
except  that  they  have  lost  the  ganoin  covering  and  are  thinner  and 
less  protective.  Cycloid  scales  have  a  smooth  circular  margin  and 
are  characteristic  of  the  older  groups  of  fishes,  while  ctenoid  scales 
have  comb-like  edges  and  are  present  mainly  in  the  higher  orders  of 
teleosts. 

The  coloration  of  fishes  is  due  to  the  presence  of  dermal  pigment- 
cells  or  chromatophores,  which  carry  variously  colored  pigments  and 
are  under  the  control  of  the  central  nervous  system.  The  more  prim- 
itive groups  of  fishes  are  colored  in  rather  neutral  fashion  and  the 
more  highly  specialized  types  are  highly  colored.  The  colors  of  trop- 
ical fishes,  especially  those  of  the  coral  reefs,  run  riot  and  rival  those 
of  the  birds  in  elaborateness  and  brilliancy.  Most  of  these  highly 
colored  fishes  belong  to  the  climax  order  of  modern  fishes,  the  Acan- 
thopterygii,  in  which  the  presence  of  high  coloration  is  taken  as  evi- 
dence of  the  onset  of  racial  senescence.  The  flounders  are  the  "  cha- 
meleons" among  fishes.  They  are  perhaps  among  all  animals  the 
most  efficient  in  their  ability  to  modify  their  color  patterns  in  re- 
sponse to  varied  backgrounds. 

CLASSIFICATION   OF  PISCES 

SUB-CLASS  I.    ELASMOBRANCHII. 

Order  I.  Pleuropterygii  (Family.  Cladoselachidae) — extinct. 
Order  II.  Ichthyotomi  (Family.  Pleuracanthidse) — extinct. 
Order  III.  Acanthodei  (Family  1.  Diplacanthidae) — extinct. 

(Family  2.  Acanthodidae) — extinct. 


PISCES  111 

Order  IV.  Plagiostomi 
Sub-Order  1.     Selachii  (12  living  and  3  extinct  families  of  sharks 

and  dog-fishes). 

Sub-Order  2.     Batoidei      (Saw-fishes,  skates,   rays   and   torpe- 
does, 7  families). 
Order  V.   Holocephali  (Chimaeras,  3  extinct  and  1  living  family). 

SUB-CLASS  II.    TELEOSTOMI 
Order  I.  Crossopterygii 

Sub-Order  1.     Osteolepida    (4  extinct  families). 
Sub-Order  2.     Cladista    (recent  genera,    Polypterus    and    Cal- 
amichihys) . 

Order  II.  Chondrostei     (5  extinct  and  two  living  families,  includ- 
ing paddle-fishes  and  sturgeons.) 

Order  III.  Holostei          (6  extinct  and  2  living  families,  including 

bow-fins  and  gar-pikes). 

Order  IV.  Teleostei. 

Sub-Order  1.  Malacopterygii  (21  families,  including  tarpons, 

herrings,  salmon,  etc.). 

Sub-Order  2.  Ostariophysi  (6  families,  including  carp,  tench, 

cat-fishes,  etc). 

Sub-Order  3.     Symbranchii  (2  families). 

Sub-Order  4.     Apodes   (5   families  of  eels). 

Sub-Order  5.  Haplomi  (14  families,  including  pickerel  and  killi- 

fishes,  etc.). 

Sub-Order  6.     Heteromi   (5  families,  Fierasfer,  etc.). 

Sub-Order  7.  Catosteomi  (11  families,  including  stickle-backs, 

pipe-fishes,  sea-horses,  etc). 

Sub-Order  8.  Percesoces  (12  families,  including  Belone,  sand- 
eels,  rag-fishes,  etc.). 

Sub-Order  9.     Anacanthini     (3  families,  including  cod,  etc.). 

Sub-Order  10.  Acanthopterygii  (78  families,  including  a  large 

proportion  of  our  commonest  fishes — 
perch,  bass,  mackerel,  flounders,  gobies, 
shark-suckers,  etc.). 

Sub-Order  11.  Opisthomi  (1  family — eel-like  fishes). 

Sub-Order  12.  Pediculati  (5  families,  including  the  Anglers, 

Bathymal  Sea-Devils,  etc.). 


112  VERTEBRATE   ZOOLOGY 

Sub-Order  13.  Plectognaihi    (7   families,     including     file-fishes, 

trunk-fishes,    puffers,    porcupine    fish, 
and  sun-fish). 
SUB-CLASS  III.      DIPNEUSTI  (Dipnoi)  Lung-Fishes. 

X2  extinct  and  2  living  families,  includ- 

Johh   bo^|  [  ing  Neoceratodus,    Protoperus,  and 

Lepidosiren). 

APPENDIX  (TO  TRUE  FISHES) 

I.  PAL,EOSPONDYLn>,E     (1  family,  between  cyclostomes  and  fishes). 
II.  OSTRACODERMI         .    (3  orders  of  8  families,  mostly  armored 

fishes). 

III.  ANTIARCHI  (1  family  of  mailed  fishes).   - 

IV.  ARTHRODIRA  (1  family  of  mailed  fishes).  3^ 

"'     ':'  ', 
It  will  be  of  interest  to  note  that  of  the  226  families  of  true  fishes 


listed  in  the  above  classification,  ~i7  2  belong  to  the  order  Teleostei. 
There  are  32  families  of  Elasmobranchii,  9  of  which  are  extinct.  The 
remaining  22  families  are  divided  among  the  ganoids  and  dipnoans, 
Although  the  Teleostei  are  unquestionably  the  characteristic 
fishes  of  the  present  and  may  be  considered  the  dominant  creatures  of 
the  waters  of  modern  times,  the  elasmobranchs,  represented  most 
typically  by  the  sharks,  are  still,  as  they  have  been  since  Devonian 
times,  a  powerful,  predaceous  tribe,  that  has  ruled  the  waters  through 
strength  and  savagery;  though  not  characterized  by  such  sheer  num- 
bers nor  by  so  many  specializations  and  adaptations  as  are  the  teleosts. 

SUB-CLASS  I.     EL/SMOBRANCHII 

The  present  day  sharks,  though  not  as  primitive  as  the  extinct 
types  of  elasmobranchs,  are  relatively  a  conservative  group.  Though 
they  have  come  through  millions  of  years  of  evolution  some  of  the 
sharks  (Fig.  67)  notably  the  Notidanidae  and  the  dog-sharks  and  rela- 
tives, are  still  very  generalized  aquatic  vertebrates  and  serve  well 
to  illustrate  primitive  vertebrate  morphology.  On  that  account  they 
are  used  extensively  as  a  main-line  vertebrate  type  (stem-type)  in 
courses  in  comparative  anatomy.  Adaptive  radiation  has  taken 
place  among  the  elasmobranchs  less  than  in  other  groups.  For  the 
most  part  they  have  retained  the  elongated  spindle-shaped  body,  the 
lightly  armored  integument,  and  active  predaceous  habits,  character- 


PISCES  113 

istic  of  plastic  or  ever-juvenile  races.  The  broad,  flat  types  such  as 
skates,  rays  and  torpedoes,  illustrate  the  bottom-feeding,  compara- 
tively sluggish  adaptive  complex,  but  this  has  not  reached  its  extreme; 
for  though  these  forms  are  more  heavily  armed  with  spines  and  small 
dermal  plates  than  are  the  sharks,  none  of  them  have  acquired  really 
heavy  armor,  nor  a  definitely  sedentary  habit. 

A  TYPICAL  ELASMOBRANCH 

The  dog-fish  Squalus  acanthias  (Fig.  61)  may  be  taken  as  a  typical 
elasmobranch  in  order  to  introduce  the  group.  These  rather  small 
predaceous  sharks  are  called  by  the  natives  of  our  Atlantic  coast 
" Horned  Dogs"  or  " Spiny  Dogs"  to  distinguish  them  from  the  sim- 
ilar but  smoother  dog-fish,  Mustelus,  which  belongs  to  a  different 
family.  In  Buzzards  Bay  and  Vineyard  Sound  the  species  was  a  few 
years  ago  so  abundant  as  to  be  a  real  pest  and  they  were  caught  in 
large  quantities  and  used  only  for  fertilizer.  They  have  recently  been 
discovered  to  be  an  excellent  food  fish  and  are  now  being  put  on  the 


FIG.  61. — The  dogfish  shark,  Squalus  acanthias.    (From  Hegner,  after  Dean.) 

market  in  canned  form  under  the  name  of  "gray  fish."  They  rove 
the  coastal  waters  in  schools  and  destroy  large  numbers  of  smaller 
fishes,  squids,  ctenophores,  and  many  worms.  They  are  caught  in 
fish  traps  where  with  their  sharp  teeth  they  do  a  good  deal  of  damage 
to  the  mesh.  Squalus  is  viviparous,  giving  birth  to  young  "pups" 
upwards  of  six  inches  in  length.  The  figures  used  to  illustrate  the 
anatomy  of  the  dog-fish,  are  for  good  reasons,  not  of  the  species  likely 
to  be  used  in  the  laboratory,  but  correspond  sufficiently  closely  with 
descriptions. 

External  Characters  (Fig.  61).— The  body  is  submarine-shaped, 
sharp  at  both  ends.  The  steering  and  balancing  organs  consist  of 
two  median  dorsal  fins,  two  pairs 'of  lateral  fins,  pectoral  and  pelvic, 
the  latter  of  which  are  in  the  male  provided  with  stiff  specialized 
portions  known  as  claspers,  used  for  holding  the  female  during  cop- 


114  VERTEBRATE  ZOOLOGY 

ulation  and  for  introducing  sperm  into  the  latter's  oviducts.  The 
eggs  are  internally  fertilized  and  embryonic  development  takes  place 
in  the  uterus  of  the  mother.  This  is  a  highly  specialized  character, 
as  will  be  clear  when  the  more  primitive  sharks  are  described.  The 
mouth  is  ventrally  situated  and  is  armed  with  a  number  of  rows  of 
teeth  on  both  upper  and  lower  jaws.  The  teeth  are  obviously  modi- 
fied placoid  scales.  The  body  is  covered  all  over  with  small  scales 
(placoid  scales),  with  a  bony  plate-like  base  of  dermal  origin  and  a 
sharp  protruding  spine  covered  with  hard  enamel  of  ectodermal  ori- 
gin. Between  the  mouth  and  the  pectoral  fins  are  the  gill-slits 
(pharyngeal  clefts)  each  of  which  opens  separately  to  the  exterior. 
The  anterior  gill-slit  on  each  side  is  small  and  modified  into  a  spiracle, 
situated  just  back  of  the  eye.  Several  external  features  discussed  in 
other  connections  are  nasal  apertures,  cloaca,  lateral-line  organs,  and 
lastly,  the  tail,  which  is  provided  with  a  typical  heterocercal  caudal  fin. 

The  Skeletal  System. — The  entire  skeleton  is  cartilaginous  with 
only  a  slight  impregnation  of  calcareous  matter.  The  cranium  is  a 
chondrocranium,  a  solid,  one-piece  capsule  completely  inclosing  the 
brain  and  the  principal  sense  organs.  The  cranium  proper  is  fused 
with  paired  nasal  capsules,  and  paired  auditory  capsules.  The 
vertebral  column  consists  of  a  series  of  hour-glass-shaped  vertebrae, 
with  lens-shaped  pieces  of  the  original  notochord  between  adjacent 
vertebrae,  and  connected  with  each  other  by  a  strand  of  notochordal 
tissue  perforating  the  entire  set  of  vertebrae  like  the  string  through 
a  chain  of  beads.  Closely  associated  with  the  skull  but  not  fused  with 
it,  is  the  mandibular  skeleton,  consisting  of  an  upper  jaw  (palato- 
quadrate  cartilage)  and  a  lower  jaw  (Meckel's  cartilage).  Back  of  the 
jaws  are  the  visceral  arches,  that  are  composed  of  upper  and  lower 
parts  like  the  jaws;  the  first  pair  being  specialized  as  the  hyoid  arch, 
the  five  others  being  the  more  generalized  branchial  arches  that  afford 
support  for  the  gills.  The  fins  all  have  cartilaginous  ray -like  supports, 
and  the  pectoral  and  pelvic  limb  skeletons  are  supported  upon  sim- 
ple horseshoe-shaped  girdles  (pectoral  and  pelvic  girdles)  each  com- 
posed of  but  one  piece  of  cartilage. 

The  Alimentary  System. — The  mouth  opens  directly  into  the 
capacious  pharnyx,  which  is  perforated  by  five  gill-clefts  and  the  paired 
spiracles.  A  short  oesophagus  of  large  caliber  leads  into  a  U-shaped 
stomach,  which  in  turn  communicates  through  a  valvular  opening,  con- 
trolled by  a  sphincter  muscle,  with  the  intestine  (Fig.  62) .  The  latter 


PISCES 


115 


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116  VERTEBRATE   ZOOLOGY 

is  short  but  of  large  diameter  and  has  a  secreting  surface  greatly  en- 
larged by  a  fold,  in  the  shape  of  a  spiral  staircase,  called  the  spiral 
valve  (a  primitive  fish  character).  Into  the  intestine  empties  the 
large  bilobed  liver  which  is  provided  with  a  gall-bladder  and  a  bile-duct. 
A  diffuse  pancreas  also  pours  its  secretion  into  the  intestine. 

The  Respiratory  System. — Branchial  respiration  is  carried  on 
in  the  six  pairs  of  branchial  clefts.  These  branchiae  are  primitive  res- 
piratory organs  consisting  of  mere  diverticula  of  mucous  membrane, 
richly  vascular  and  supported  by  cartilaginous  processes,  called  gill- 
rays.  The  water  enters  the  mouth  and  is  forced  out  through  the  gill- 
slits.  In  doing  so,  it  aerates  the  gill-filaments  and  provides  oxygen 
for  the  blood  that  circulates  rapidly  through  them. 

The  Circulatory  System  (Fig.  63). — The  architecture  of  this 
system  is  in  the  main  laid  out  in  accord  with  the  branchial  system. 
The  heart  receives  only  venous  blood  from  a  single  precaval  vein, 
and  pumps  it  forward  in  a  common  ventral  aorta,  which  divides  into 
five  pairs  of  afferent  branchial  arteries,  each  of  which  carries  blood  to 
one  set  of  branchiae.  A  corresponding  efferent  branchial  vessel  picks 
up  the  aerated  blood  from  each  gill  and  carries  it  to  a  dorsal  aorta 
which  in  turn  distributes  the  blood  to  all  parts  of  the  body  both  an- 
terior^ and  posteriorly.  A  complex  system  of  veins  consisting  of  a 
general  systemic  part,  a  hepatic  portal,  and  a  renal  portal  system,  re- 
turns the  blood  to  the  heart  along  paired  channels  called  anterior 
and  posterior  cardinal  veins. 

Urogenital  System. — The  nephridia  (kidneys)  consist  of  paired 
strap-like  organs  lying  side  by  side  along  the  roof  of  the  body  cavity. 
Microscopic  examination  shows  that  these  long  organs  are  composed 
of  paired  nephric  tubules  each  of  which  opens  at  one  end  into  the  cce- 
lom  and  at  the  other  into  a  nephric  duct  that  leads  to  the  cloaca.  The 
functional  adult  kidney  is  a  mesonephros  or  " mid-kidney,"  the  pro- 
nephros  being  reduced  to  a  mere  vestige,  though  functional  in  the 
larva.  The  testes  are  paired  whitish  bodies  of  rather  flat,  long  and 
narrow  shape,  that  communicate  with  the  cloaca  by  paired  ducts, 
the  vasa  deferentia.  The  ovaries  are,  when  mature,  large  lobulated 
bodies  attached  to  the  dorsal  anterior  part  of  the  body  cavity  and 
communicating  with  the  exterior  by  large  paired  oviducts,  which 
unite  anteriorly  and  have  a  single  funnel-like  opening-  for  receiving 
the  large  ova.  Each  oviduct  is  regionally  modified  into  a  shell  gland 
and  a  uterus. 


PISCES 


117 


118 


VERTEBRATE  ZOOLOGY 


Nervous  System.  —  Though  the  brain  (Fig.  64)  is  very  small,  it  is 
larger  in  proportion  to  body  size  than  that  of  the  Cyclostomata.  The 
most  striking  feature  is  the  large  size  of  the  olfactory  bulbs.  Cerebral 

hemispheres  are  well  defined,  cerebellum 
is  large  and  overlaps  anteriorly  a  part  of 
the  optic  lobes  and  posteriorly  a  part  of 
the  medulla  oblongata.  The  region  of 
the  thalamencephalon  from  which  come 
the  optic  nerves  is  comparatively  small 
and  slender.  The  spinal  cord  is  typical, 
and  inclosed  within  cartilaginous  neural 
arches. 

Sense  Organs.  —  The  dominant  sense 
of  the  elasmobranch  is  olfactory;  the 
sense  organ  consisting  of  large  con- 
voluted invaginations  in  close  contact 
with  the  olfactory  bulbs  of  the  brain. 
The  eyes,  though  small  and  probably 
not  especially  keen-sighted,  are  well  de- 
veloped and  connected  with  the  brain 
by  rather  slender  optic  nerves.  The 
auditory  organs  are  inclosed  in  cartila- 
ginous capsules  and  consist  of  three 
,  ^circular  canak,  a  utrMus,  and  a 

vew.    2,  pineal  stalk;  3,  olfac-  small  simple  sacculus.    The  lateral  line 
tory   lobe;    4,    cerebral   hemi-  sense  organs  are  in  grooves  of  the  skin 

f^o  tbttSS;  »<*>  <*»*&<*  d°^  ***  divide  »*> 

10,  roof  of  hind-brain;  11,  12,  several   branches   in   the   head  region, 

13,  14,  muscles  that  move  the  One  above  and  one  below  the  eye  and 

SSu&'S'Urii?;  ««»  »  *>  hyo-mandibular  region. 

17,  main  trunk  of  vagus  nerve;  The   dog-fish   represents   neither   ex- 

II-X,  roots  of  the  cranial  nerves,  treme  of  elasmobranch  evolution,   but 


primitive  extinct  sharks  and  the  most 
specialized  modern  skates  and  rays.  It  will  be  instructive  to  con- 
sider some  of  the  most  primitive  elasmobranchs  in  order  to  be  able 
to  judge  more  certainly  which  of  the  characters  of  our  favorite  labo- 
ratory fish  are  specialized  and  which  are  still  primitive.  A  very  prim- 
itive form  of  living  shark  is  the  Frilled  Shark,  Chlameidoselachus  (Fig- 


PISCES 


119 


67,  A).  Its  terminal  mouth  and  almost  diphycercal  caudal  fin  are 
decidedly  archaic.  Whether  its  frilled  branchial  clefts  are  primitive 
or  specialized  it  is  impossible  to  say. 

EXTINCT  ELASMOBRANCHS 

Some  of  the  extinct  families  of  sharks:  Cladoselachidse,  Pleura- 
canthidse,  Diplacanthidae,  and  Acanthodidae,  bring  to  light  certain 
conditions  that  are  obviously  more  primitive  than  those  of  any  of  the 
modern  elasmobranchs.  They  all  have  the  mouth  terminal  instead 
of  ventral,  indicating  that  the  terminal  mouth  which  characterizes 
most  of  the  Teleostomi  is  more  primitive  than  the  ventral  mouth 


FIG.  65. — Cladoselache  fyleri;  Upper  Devonian,  Ohio.    A,  right  side  view;  B, 
ventral  view;  C,  front  view;  restored.     (From  Lankester,  after  Woodward.) 

of  the  modern  elasmobranchs.  There  are  no  claspers  on  the  ven- 
tral (pelvic)  fins,  indicating  that  the  habit  of  copulation,  with  internal 
incubation  of  eggs  and  consequent  viviparity,  so  common  among 
modern  elasmobranchs,  is  a  caenogenetic  specialization.  The  noto- 
chord  persists  as  an  unbroken  elastic  rod  and  the  neural  and  haemal 
arches  are  developed  only  about  as  far  as  they  are  in  the  Cyclostomata. 
The  exoskeletal  elements  appear  to  be  wanting  in  Pleuracanthidce  and, 
in  the  other  families,  are  smaller  and  less  developed  than  in  modern 
sharks. 

Each  of  these  families  shows  some  one  or  two  features  more  prim- 
itive than  the  others.    One  of  the  most  striking  features  shown  by 


120 


VERTEBRATE   ZOOLOGY 


-s 


these  primitive  sharks  is  the  paired  fin  system 
of  Cladoselache,  a  fish  that  on  the  whole  is 
generally  admitted  to  be  the  most  primitive 
of  all  true  fishes,  though  its  remains  were 
found  in  the  Devonian  rocks  millions  of  years 
later  than  the  earliest  ostracoderms.  The 
paired  fins  of  Cladoselache  (Fig.  65)  are  so- 
called  " lappet-fins,"  broad  based  and  closely 
resembling  in  bony  framework  the  median 
fins,  such  as  the  dorsals.  The  skeleton  of  these 
fins  consists  of  two  sets  of  elements.  The 
slender,  nearly  parallel,  un jointed  cartilages 
occupy  a  distal  position,  and  proximal  to  these 
is  a  less  numerous  set  of  shorter  and  stouter 
cartilages  imbedded  in  the  body  wall  and  cor- 
responding to  the  large  cartilages  (propte- 
yrgium,  mesopterygium,  and  metapterygium) 
of  the  modern  shark.  The  pectoral  fin  of 
Cladoselache  is  not  quite  so  primitive  as  the 
pelvic  and  furnishes  a  transition  between  the 
latter  and  the  fins  of  modern  elasmobranchs. 
The  location  and  general  arrangement  of  the 
paired  fins  of  Cladoselache  have  given  rise  to 
the  theory  that  "the  fins  of  fishes  arise  from 
lateral  skin  folds  of  the  body,  into  which  are 
extended  internal  stiffening  rods."  These 
folds  are  supposed  to  be  essentially  like  those 
composing  the  median  fin  system  and  are  be- 
lieved to  have  been  at  one  time  continuous 
with  them,  as  in  the  hypothetical  case  de- 
scribed by  Dean.  This  continuous  fold  is  sup- 
posed to  have  been  specialized  in  two  regions 
to  form  the  pelvic  and  pectoral  enlargements 
and  to  have  disappeared  in  between. 

FIG.  66. — Pleuracanthus  ducheni,  restored.  A',  ventral  fin;  B,  basal  fin- 
cartilages;  D,  dermal  margin  of  fin;  D  S.,  dermal  fin-spine;  H.  A,  haemal  arches; 
HM,  hyo-mandibular;  I.  C,  inter-neural  plates;  M.  C,  Meckel's  cartilage;  N, 
notochord;  NA,  neural  process  and  spine;  P,  supposed  pelvic  cartilage;  PQ, 
palatoquadrate;  R,  radial  fin-cartilages;  R',  ribs;  S.  G,  shoulder  girdle.  (From 
Parker  and  Haswell,  after  Dean.) 


PISCES  121 

Pleuracanthus  (Fig.  66)  also  contributes  a  very  primitive  fin  charac- 
ter but  in  another  system,  the  caudal.  Instead  of  having  the  hetero- 
cercal  type  of  tail  fin  which  is  characteristic  of  modern  elasmobranchs, 
it  has  a  still  more  primitive  one,  a  continuous  fin-fold  of  the  diphy- 
cercal  type,  running  smoothly  about  the  tail.  This  is  the  type  that 
one  finds  in  Amphioxus,  in  the  hag-fishes,  and,  in  a  slightly  modified 
form,  in  the  lampreys.  It  is  also  the  earliest  embryonic  stage  in  the 
median  fin  development  of  teleosts.  There  seems  little  question,  then, 
that  the  ancestral  elasmobranchs  had  this  kind  of  caudal  fin. 

Putting  together  the  most  primitive  characters  of  all  of  these  ex- 
tinct elasmobranchs  we  are  able  to  describe  a  hypothetical  ancestral 
shark  which  is  also  probably  the  prototype  of  the  earliest  real  verte- 
brate. 

THE  HYPOTHETICAL  ANCESTOR  OF  THE  ELASMOBRANCHS,  AND  OF 
FISHES  IN  GENERAL 

This  creature  must  have  had  an  elongated,  spindle-shaped,  fusi- 
form body  with  terminal  mouth,  armed  with  dermal  teeth,  with  prob- 
ably more  than  seven  gill-slits,  with  small  lozenge-shaped  dermal 
denticles  scattered  over  the  skin,  with  lappet-like  paired  fins,  and 
a  diphycercal  tail-fin  with  low  dorsal  specializations  of  this  fin-fold. 
Internally,  it  probably  had  a  persistent  notochord  with  the  merest 
vestiges  of  vertebral  arches.  It  also  doubtless  had  lateral  line  organs 
in  open  grooves,  and,  having  no  claspers,  laid  small  eggs  in  the  open 
sea.  The  intestine  probably  had  at  least  a  primitive  spiral  valve. 

SOME  OF  THE  SPECIALIZED  MODERN  ELOSMOBRANCHS  OF  THE  ORDER 

PLAGIOSTOMI 

Sub-order  1 .  Selachii  (true  sharks)  have  remained  on  the  whole  com- 
paratively unspecialized.  For  the  most  part  they  are  active,  free- 
swimming,  predaceous  creatures  such  as  the  ancestral  sharks  must 
have  been.  Among  the  more  striking  members  of  the  Selachii  are  the 
Hammer-Heads,  the  Whale-Sharks,  and  the  Angel-Sharks. 

The  Hammer-Heads  (Sphyrnidce)  are  characterized  by  the  lateral 
protrusion  of  the  eyes  on  large  flat  stalks  (Fig.  67,  D)  supported  by 
cartilaginous  extensions  of  the  cranium.  There  are  all  gradations 
between  the  only  slightly  extended  eyes  to  those  in  which  the  eyes  ex- 
tend so  far  as  to  give  a  width  five  times  that  of  the  normal  head.  It 
is  not  known  that  these  greatly  projecting  eyes  are  of  any  especial 


122 


VERTEBRATE   ZOOLOGY 


use  to  their  possessors.  It  would  be  very  difficult  therefore  to  ex- 
plain their  origin  on  a  natural  selection  basis.  Those  who  have  worked 
experimentally  with  fish  embryos  have  often  observed  in  abnormal 
embryos  tendencies  for  the  eyes  to  project  on  stalks,  and  it  may  well 


FIG.  67.— Group  of  Sharks  (Selachii).  A,  Frilled  Shark  (Chlameidoselachus 
anguineus)  after  Giinther.  B,  Female  Dog-Fish  (Scyllium  canescens),  after 
Glinther.  C,  Thresher  Shark  (Alopecias  vulpes);  after  Jordan  and  Evermann. 
D,  Hammer-head  shark  (Sphyrna  zygana),  male,  after  Bridge.  E,  Angel  Shark 
(Rhina  squatind),  after  Bridge. 

be  that  some  similar  explanation  would  account  for  this  specific  char- 
acter in  the  Hammer-Heads. 

The  Whale-Sharks  (Rhinodontidce)  are  of  interest  because  they 
are  much  the  largest  true  fishes  that  have  ever  lived.  They  are  said 
sometimes  to  exceed  fifty  feet  in  length  and  to  be  of  proportionate 
bulk.  Such  a  shark  would  be  able  easily  to  swallow  a  man,  but  it 


PISCES  123 

never  does.  Instead,  it  feeds  only  upon  small  pelagic  animals,  in- 
cluding fishes,  squids,  and  other  small  forms,  which  it  strains  out  of 
the  water  by  means  of  the  fringes  on  its  long,  slender  gill-rakers. 

The  Thresher  Shark  (Fig.  67,  C)  is  remarkable  chiefly  for  the 
great  length  of  the  upper  lobe  of  the  caudal  fin,  which  equals  the  rest 
of  the  body  in  length. 

Angel-Sharks  (Rhinidce)  constitute  an  interesting  transition  be- 
tween the  Selachii  and  the  Batoidei  (skates,  rays,  etc),  in  that  they 
have  a  short  broad  form  (Fig.  67,  E)  with  marked  lateral  expansions 
of  the  pectoral  and  pelvic  fins  that  look  like  wings  and  give  the  group 
its  name.  In  habits  they  are  more  like  the  rays  than  the  sharks,  in 
that  they  frequent  the  bottom  and  do  not  follow  the  free-roving  life 
of  the  typical  sharks. 

Sub-order  2.  Batoidei  (skates,  rays,  torpedoes),  are  all,  with  the  single 
exception  of  the  Saw-Fishes  (Pristidce),  rhombic  or  discoidal  in  form, 
due  to  the  dorso-ventral  flattening  of  the  body  and  the  excessive 
growth  of  the  pectoral  fins.  For  the  most  part  they  are  sluggish 
bottom-feeders  that  swim  slowly  over  the  sea-bottom  at  various 
depths,  using  the  pectoral  fins  as  propellers,  waves  of  propulsion 
passing  from  in  front  backward.  They  are  protectively  colored  on 
top  so  as  closely  to  resemble  the  sea-bottom,  but  are  usually  white  or 
uniformly  pinkish  below.  The  tail  is  weak  and  of  little  use  in  locomo- 
tion, being  used  merely  as  a  rudder  or,  in  the  sting  rays,  as  a  weapon  of 
defense.  The  typical  members  of  the  Batoidei  are  the  common  skates 
(Fig.  68,  A) .  These  fishes  have  an  almost  perfect  rhomboidal  outline, 
resembling  a  broad  kite  with  a  short  tail.  They  catch  their  prey 
(fishes,  crustaceans,  etc.)  by  dropping  suddenly  upon  it  and  covering 
it  with  the  broad  body  and  fins.  The  food  is  ingested  by  means  of  the 
ventrally  placed  mouth  armed  with  rasping  file-like  teeth.  Some  of 
the  largest  species  reach  a  diameter  of  as  much  as  seven  or  eight  feet. 

The  Electric  Rays  (Torpedinidce)  are  more  nearly  circular  in 
body  outline  (Fig.  68,  C)  than  the  skates.  They  are  especially  note- 
worthy on  account  of  the  presence  of  paired  electric  organs,  developed 
from  two  pillars  of  muscle  situated  between  the  pectoral  fins.  They 
are  capable  of  giving  at  will  quite  a  heavy  electric  shock.  This  mode 
of  defense  is  in  accord  with  the  entire  absence  of  dermal  spines,  for 
a  fish  capable  of  giving  a  shock  needs  no  armor. 

Sting  Rays  or  Whip-Tailed  Rays  (Fig.  68,  D).— These  tropical 
Rays  are  especially  noted  for  the  long  flexible  tail  armed  with  one  or 


124 


VERTEBRATE  ZOOLOGY 


more  serrated  spines  in  the  position  of  a  dorsal  fin.  These  spines 
which  may  be  eight  or  nine  inches  long  are  capable  of  inflicting  very 
severe  wounds,  which  become  infected  or  poisoned  by  having  intro- 
duced into  them  the  mucous  secretions  that  bathe  the  cutting  spines. 


D  £ 

FIG.  68. — Group  of  Batoidei.  A,  skate,  Raia  batis,  male,  ventral  view  (after 
Hertwig).  B,  Saw-Fish,  Pristis  antiquorum,  (after  Cuvier).  C,  Electric  Ray, 
Torpedo  ocellata  (after  Bridge).  D,  Sting-Ray,  Stoasodon  narinari,  (after  Jordan 
and  Evermann).  E,  Eagle  Ray,  Myliobatis  aquila  (after  Bridge). 

Eagle  Rays  (Myliobatidce)  show  extremely  pronounced  specializa- 
tion of  the  pectorial  fins  (Fig.  68,  E),  giving  the  body  a  considerably 
greater  breadth  than  length,  the  width  being  sometimes  as  great  as 


PISCES  125 

twenty  feet.  They  catch  their  prey  by  enveloping  it  in  their  great 
" wings."  They  are  true  " sea-vampires,"  dreaded  by  pearl  divers 
near  Panama,  who  are  said  to  have  been  caught  and  drowned  by  these 
great  " winged"  creatures.  They  are  sometimes  known  as  "devil 
fishes." 

Saw-Fishes  (Pristiidce)  exhibit  one  of  the  most  striking  special- 
izations seen  among  the  Batoidei.  In  them  the  body  (Fig.  68,  B)  is 
only  slightly  broadened  laterally,  but  the  rostrum  is  prolonged  until 
it  reaches  a  length  half  as  great  as  the  rest  of  the  body.  The  rostrum  is 
armed  with  two  lateral  rows  of  knife-like  teeth  which  enable  the  fish 
to  deal  vicious  slashing  blows  at  its  enemies.  It  is  said  that  they  at- 
tack whales  in  the  soft  parts  behind  the  flippers,  tearing  off  and  de- 
vouring pieces  of  flesh. 

ORDER  HOLOCEPHALI  (CHIMERAS) 

Chimseras  (Fig.  69)  are  by  some  considered  as  a  divergent  offshoot 
of  the  Elasmobranchii;  by  others  they  are  placed  in  a  distinct  sub- 
class of  coordinate  value  with  the  whole  sub-class  Elasmobranchii. 
It  is  difficult  to  decide  between  these  two  alternatives.  There  are 
undoubtedly  some  characters  that  relate  the  Holocephali  to  the  Elas- 
mobranchii, but  there  are  also  some  very  fundamental  differences. 
They  agree  with  the  elasmobmnchs  in  the  following  ways:  a  wholly 
cartilaginous  endoskeleton;  no  cartilage  or  membrane  bones;  the 
limb  girdles  and  the  limb  skeletons  essentially  elasmobranch  in  struc- 
ture; the  dermal  denticles,  present  locally  in  some  modern  forms 
and  more  generally  in  extinct  forms,  agree  with  those  of  elasmo- 
branchs;  the  brain  is  very  similar;  the  reproductive  system,  including 
clasping  organs  in  the  male,  and  large  chitinous-shelled  eggs,  remind 
one  strongly  of  the  elasmobranchs;  there  is  no  air-bladder;  there  is  a 
spiral  valve;  there  is  a  conus  arteriosus;  the  nostrils  are  connected 
with  mouth  by  oro-nasal  grooves.  The  group  is  undoubtedly  one  of 
great  antiquity  and  probably  branched  off  from  the  elasmobranchs 
of  early  times.  The  specialized  features  are:  The  skull  is  autostylic, 
the  jaw  cartilage  (palatoquadrate)  being  firmly  fused  with  the  base  of 
the  cranium,  a  character  which  accounts  for  the  name  (holos — whole 
or  undivided;  cephalos—head) .  The  teeth  are  modified  into  large 
crushing  dental  plates.  The  claspers,  instead  of  consisting  merely  of 
one  pair  derived  from  the  pelvic  fins,  are  five  in  number.  One  pair 
is  like  that  of  the  elasmobranchs;  a  second  pair  occurs  in  pockets  of 


126 


VERTEBRATE  ZOOLOGY 


the  skin  in  front  of  the  pelvic  fins,  and  a  median  finger-like  process  is 
hinged  to  the  forehead  between  the  eyes.  Just  how  these  claspers 
are  used  is  not  known.  They  also  show  certain  tendencies  in  the 


FIG.  69. — Group  of  Holocephali  (Chimaeras).  A,  Chimcera  monslrosa  (after 
Bridge).  B,  Callorhynchus  antarcticus,  male  (after  Parker  and  Haswell).  G, 
Harriotta  raleighana  (after  Goode  and  Bean). 

direction  of  the  Teleostomi,  such  as  the  crowding  together  of  the 
branchial  arches  underneath  the  head,  the  development  of  an  oper- 
culum  covering  all  but  the  last  gill-slit,  the  suppression  of  the  spir- 
acles, and  the  absence,  of  a  cloaca. 
These  curious  fishes  are  of  moderate  size,  one  to  three  feet  in  length. 


PISCES  127 

They  inhabit  the  comparatively  deep  seas,  ranging  from  200  to  1,200 
fathoms,  though  one  species,  Chimcera  colliei,  lives  at  or  near  the 
surface.  In  all  of  them  the  median  fin  system  has  two  peculiarities; 
a  strong  spine  anterior  to  the  front  dorsal  fin  and  long  whip-like  tail 
with  either  a  diphy cereal  or  a  slightly  developed  heterocercal  fin.  The 
Holocephali  may  be  dismissed  then  as  an  interesting  but  not  especially 
significant  side-line  of  elasmobranch  evolution. 

SUB-CLASS  II.    TELEOSTOMI 

This  great  group  of  fishes  to  which  belong  nearly  all  of  the  modern 
species  except  the  Elasmobranchii,  consists  of  a  few  fragmentary 
vestiges  of  formerly  abundant  fish  faunas  and  a  vast  assemblage  of 
modern  bony  fishes.  The  lowest  of  the  vestigial  orders,  the  Crossop- 
terygii  and  the  Chondrostei,  show  many  evidences  of  relationship 
with  the  primitive  sharks  and  very  probably  descended  from  some 
primitive  shark  type  at  about  the  same  time  that  the  sharks  gave  off 
the  Batoidei  and  the  Holocephali.  It  may  be  said  in  introducing  the 
Teleostomi  that  the  most  primitive  order,  Crossopterygii,  is  from  the 
standpoint  of  vertebrate  evolution  of  very  especial  importance;  for 
the  group  seems  to  belong  to  one  of  the  main-trunk-lines  of  evolution 
and  to  be  an  important  junction  point  for  several  branch-lines  of 
vertebrate  phylogeny.  It  is  now  believed  that  the  Crossopterygii  gave 
rise  not  only  to  the  other  orders  of  Teleostomi,  including  the  modern 
teleosts,  but  to  the  Dipneusti  and  to  the  first  Amphibia. 

The  principal  morphological  characters  of  the  Teleostomi  as  a  whole  are: 

1.  The  process  of  ossification  of  a  primitively  cartilaginous  endo- 
skeleton  has  resulted  in  the  appearance  of  a  number  of  separate  bones 
in  the  skull,  and  the  jaws  have  also  ossified  more  or  less  completely. 
The  roof  of  the  chondrocranium  has  been  covered  over  by  a  set  of 
dermal  investing  bones  without  teeth  or  denticles. 

2.  The  primary  jaws  (both  upper  and  lower)  have  been  covered 
over  and  reinforced  by  tooth-bearing  investing  bones,  developed  in  the 
dermis. 

3.  The  operculum  over  the  gills  is  supported  by  a  more  or  less 
elaborate  dermal  skeleton  that  becomes  rather  intimately  associated 
with  the  skull  and  lends  a  false  appearance  of  complexity  to  the  latter. 

4.  The  pectoral  girdle,  which  is  often  attached  to  the  skull,  is 
also  made  more  complex  by  the  addition  of  investment  bones  from 
the  dermis.    The  pelvic  girdle  is  usually  absent  or  vestigial 


128  VERTEBRATE  ZOOLOGY 

5.  The  vertebral   column  is   not  very  compact,   the  vertebra 
being  often  without  a  centrum;  if  the  latter  is  present  it  is  an  arch- 
centrum. 

6.  The  fins  are  supported  by  bony  dermal  rays. 

7.  The  integument  is  characterized  by  scales  without  the  denticle 
or  spike  seen  in  the  placoid  type  of  scale.    They  are  ganoid,  cyloid, 
or  ctenoid  in  form  and  may  be  either  tessellated  (laid  like  tiles)  or 
imbricated  (overlapping  like  shingles),  the  former  being  the  more 
primitive  condition. 

8.  No  claspers  are  known  in  the  group,  though  in  some  groups 
the  pelvic  fin  may  be  modified  as  an  intromittent  organ  used  in  sexual 
copulation. 

9.  Most  members  of  the  sub-class  have  an  air-bladder,  which 
serves  to  compensate  for  the  additional  weight  caused  by  the  ossified 
skeleton. 

10.  The  gill  filaments  project  beyond  the  edges  of  the  inter- 
branchial  septa. 

11.  The  nasal  sacs  have  no  naso-oral  grooves  and  they  open  by 
separate  nares. 

12.  The  brain  has  a  much  reduced  cerebrum,  with  small  olfactory 
lobes. 

13.  No  cloaca. 

14.  The  ova  are  usually  small  and  numerous  and  range,  with 
respect  to  the  cleavage,  from  holoblastic  to  meroblastic  types,  the 
more  primitive  types  resembling  the  eggs  of  Amphioxus  in  that  they 
have  holoblastic  cleavage. 

Practically  all  of  these  characters  show  an  evolution  away  from  the 
elasmobranch  condition.  In  some  cases  the  evolution  is  progressive 
and  in  others  regressive. 

ORLER   I.   CROSSOPTERYGII    (LOBE-FINNED   GANOIDS) 

This  very  important  group  which  was  abundant  in  Devonian  times 
and  is  represented  to-day  by  Polypterus  and  Calamychthys,  is  the 
most  primitive  division  of  the  Teleostomi,  and,  from  the  phylogenetic 
standpoint,  much  more  significant  than  either  of  the  other  ganoid 
orders  or  the  teleosts.  There  are  evidences  that  this  order  cf  fishes  is 
the  ancestral  group  not  only  of  all  the  higher  fishes,  but  of  the  terres- 
trial vertebrates. 


PISCES  129 

There  are  in  this  order  four  extinct  families  belonging  to  the  sub- 
order Osteolepida,  and  one  family,  represented  by  two  genera  of 
present-day  African  "lobe-fins"  of  the  sub-order  Cladista.  Both  of 
these  types  are  essentially  antique,  true  "  living  fossils,"  although  no 
real  fossils  of  these  genera  have  as  yet  been  discovered. 

A  somewhat  detailed  description  of  Polypterus  bichir  will  serve  to 
introduce  the  reader  to  the  characters  of  the  Crossopterygii  that  are 
of  most  importance. 

According  to  Harrington,  Polypterus  bichir  inhabits  the  deeper 
waters  of  the  Nile  but  does  not  bury  itself  in  the  mud  like  a  true  mud- 
fish. The  large  lobate  pectoral  fins  are  used  as  balancers  and  to  some 
extent  as  paddles  in  swimming,  but  the  most  significant  function  of 
these  fins  is  their  use  as  supports;  for  when  resting  on  the  bottom,  the 
head  is  held  up  from  the  mud  by  the  tips  of  the  fins,  much  as  a  mud- 
puppy  (Necturus)  is  supported  by  its  fore  legs.  The  excellent  figure 
(Fig.  102,  A),  after  Osborn,  shows  these  fins  used  as  supporting  limbs. 
Such  a  habit  suggests  the  mode  of  origin  of  the  terrestrial  limb  from 
the  aquatic  limb.  The  air-bladder  in  this  primitive  fish  is  not  merely 
or  principally  a  hydrostatic  organ,  but  is  an  accessory  respiratory 
organ.  It  is  connected  by  a  primitive  trachea  with  the  pharynx  and 
is  used  as  a  lung.  The  supporting  character  of  the  pectoral  fin  and 
the  primitive  lung  are  considered  of  especial  importance  as  furnishing 
the  beginnings  of  adaptations  for  terrestrial  life. 

The  larva  of  Polypterus  is  interesting  on  account  of  its  similarity 
to  amphibian  larvae.  Budget  has  described  a  larva  of  P.  senegalus 
(Fig.  70,  C)  as  quite  a  striking  object,  beautiful  in  color  and  markings. 
Its  most  remarkable  feature  is  the  single  pair  of  external  gills,  which 
are  pinnate  in  structure  and  are  evidently  homologous  with  the  exter- 
nal gills  of  amphibian  larvae  and  those  of  the  perennibranchiate 
urodeles.  The  larva,  like  the  adult,  uses  the  pectoral  fins  as  support- 
ing appendages.  The  median  fin  system  is  of  the  primitive  diphy- 
cereal  type  like  that  of  a  tadpole,  but  differs  from  the  latter  in  having 
cartilaginous  supports  or  rays.  The  species  of  Polypterus  figured 
(Fig.  70,  A)  is  P.  senegalus. 

The  other  living  genus  of  Crossopterygii,  Calamichthys  (Fig.  70,  B) 
is  much  like  Polypterus,  but  is  a  greatly  elongated  eel-like  edition 
of  the  latter.  It  is  confined  to  middle  western  Africa,  living  in  the 
small  muddy  rivers  of  that  region,  swimming  about  with  an  undulat- 
ing, snaky  motion  and  feeding  mainly  on  crustaceans. 


130 


VERTEBRATE  ZOOLOGY 


The  more  important  anatomical  characters  of  Polypterus  and  its 
allies  are  as  follows: 


PC/ 


FIG.  70. — Crossopterygii. — A,  Polypterus  senegalus;  s,  position  of  spiracle  (after 
Bridge).  B,  Calamichthys  calabaricus  (after  Bridge).  C,  Larva  of  Polypterus  sene- 
galus, showing  characteristic  attitude  when  resting  on  the  bottom,  and  the  large  ex- 
ternal or  cutaneous  gills  (after  Budgett).  D,  Lateral  view  of  cranium  of  Polypterus. 
a,  angulare,  ar,  articulare ;  d,  dentary ;  e,  ethmoid  ;  /,  frontal ;  m,  maxillary ;  n, 
nasal;  p,  parietal;  pm,  premaxillary ;  po,  post -orbital ;  q,  quadrate;  st,  supra- 
temporal  ;  y,  cheek  plate ;  z,  row  of  small  spiracular  ossicles.  (After  Traquair.) 

1.  Endoskeleton. — The  cartilaginous  skeleton  is  largely  ossified, 
the  chondrocranium  being  divided  into  a  number  of  distinct  bones. 


PISCES  131 

The  notochord  is  replaced  by  bony  vertebrae  which  are  hour-glass- 
shaped. 

2.  The  skull  (Fig.  70,  D)  is  covered  above  and  below  by  numer- 
ous dermal  investment  bones  that  are  much  like  those  of  the  primitive 
extinct  Amphibia  (Stegocephali) . 

3.  The  integument  is  covered  by  heavy  rhomboid  scales  that 
are  plated  externally  with  a  sheet  of  ganoin. 

4.  The  pectoral  fins  are  lobose  in  outline,  but  in  their  skeletal 
parts  show  evidences  of  relationship  to  the  pentadactyl  limb.     The 
basal  bones  are  homologous  with  the  finger,  wrist,  and  arm  bones  of 
Amphibia,  and  the  fringe  of  dermal  rays  is  the  aquatic  part  of  the 
appendage. 

5.  There  is  a  persistent  spiracle,  like  that  of  the  elasmobranchs. 

6.  The  intestine  has  a  spiral  valve. 

7.  The  conus  arteriosus  has  several  rows  of  valves. 

8.  The  median  fin  system  is  essentially  continuous  from  dorsal 
to  ventral  side,  though  a  large  part  of  the  anterior  dorsal  end  of  it  is 
broken  into  separate  spines  each  with  a  flap  of  skin  back  of  it.    The 
caudal  fin  is  diphycercal. 

9.  The  pelvic  fins  are  much  reduced. 

10.  The  air-bladder  is  double  and  opens  on  the  ventral  side  of 
the  pharynx. 

11.  The  lower  jaw  is  sheathed  in  dermal  investment  bones,  the 
dentary  and  angulare. 

Some  of  the  extinct  Crossopterygii  were  more  highly  specialized  in 
certain  respects  than  Polypterus.  They  have  the  median  and  paired 
fins  more  like  those  of  modern  teleosts,  overlapping  scales,  and  either 
heterocercal,  or  a  modified  type  of  the  latter  called  gephyrocercal, 
caudal  fins.  They  are,  however,  more  primitive  than  Polypterus  in 
having  persistent  notochord  and  acentrous  vertebrae. 

On  the  whole  then  Polypterus  may  be  said  to  be  the  most  general- 
ized teleostome  fish  living  and  may  well  be  considered  as  the  prototype 
of  that  group. 

ORDER  II.  CHONDROSTEI  (CARTILAGINOUS  GANOIDS) 

The  Chondrostei  and  Holostei  of  the  ganoid  orders,  and  the  Tele- 
ostei,  together  constitute  a  division  known  as  Actinopterygii,  in  which 
the  paired  fins  are  sharp  instead  of  lobate  as  in  the  Crossopterygii. 
The  modern  representatives  of  the  Chondrostei  are  the  Paddle-Fish 


132 


VERTEBRATE  ZOOLOGY 


or  Spoon-Bill  (Polyodon),  and  the  sturgeon  (Acipenser).  These  are 
considered  to  represent  the  culmination  of  degenerative  evolutionary 
processes,  combined  with  certain  marked  specializations. 

The  Spoon-Bill  (Fig.  71,  B)  inhabits  the  Mississippi  River  and 
its  tributaries.  It  is  a  sluggish  creature,  feeding  chiefly  on  mud  that 
it  shovels  up  with  its  spade-like  snout.  The  mud  is  strained  through 
gills  provided  with  unusually  long  gill-rakers,  which  serve  to  catch  the 
food  particles  and  let  the  mud  go  through  with  the  water  current. 
The  paddle-like  rostrum  is  richly  supplied  with  tactile  end-organs, 


FIG.  71. — Chondrostei.  A.  The  Sturgeon,  Acipenser  ruthenus  (after  Cuvier). 
B.  The  Spoon-Bill  or  Paddle-Fish,  Polyodon  folium  (after  Bridge). 

enabling  the  fish  to  detect  the  presence  of  food  in  the  mud.  An  al- 
lied genus  of  Spoon-Bill  (Psephurus)  inhabits  some  of  the  principal 
rivers  of  China  and  evidently  lives  the  same  type  of  life  as  Polyodon. 
The  shape  of  the  body  is  decidedly  selachoid  or  shark-like,  and  the 
skin  is  apparently  scaleless,  though  scattered  vestigial  scales  are 
found,  and  a  patch  of  rhombic  scales  occurs  on  the  upper  hah0  of 
the  caudal  lobe. 

The  Sturgeon  family  (Fig.  71,  A)  is  a  widely  distributed  group, 
superficially  decidedly  selachoid  in  appearance.  The  rostrum  is  pro- 
longed into  a  snout  with  a  transverse  row  of  barbels  depending  from 
its  ventral  surface.  The  mouth  is  small  and  protrusible,  but  is  with- 


PISCES  133 

out  teeth  in  the  adult.  The  scales  are  remarkable  in  that  they  are  ar- 
ranged in  five  widely  separated  longitudinal  rows  of  keeled  bony  ele- 
ments. The  dermal  bones  of  the  roof  of  the  skull  are  fused  into  a  solid 
shield.  The  sturgeons  inhabit  the  inland  lakes  and  seas,  being  found 
in  our  own  Great  Lakes,  in  the  Black  Sea,  the  Caspian  Sea,  and  the 
tributaries  of  these  lakes  and  seas.  They  feed  upon  mollusks,  worms, 
small  fishes,  and  vegetation,  the  mouth  being  protruded  as  a  cylin- 
drical spout  and  thrust  into  the  mud  in  search  of  food.  They  some- 
times reach  a  very  large  size — individuals  having  been  taken  that 
weighed  2,760  and  3,200  pounds.  The  Russian  delicacy,  caviar,  is 
made  of  the  eggs  of  sturgeons.  The  flesh  of  the  sturgeon  is  also  an 
excellent  food  and  is  largely  used. 

Many  extinct  Chondrostei  are  known,  and  in  every  case  they  are 
nearer  the  type  exemplified  by  the  Crossopterygii  than  they  are  like 
the  modern  Spoon-Bill  and  sturgeon.  It  is  therefore  highly  probable 
that  the  group  was  derived  from  the  Crossopterygii. 

ORDER  III.    HOLOSTEI 

This  rather  large  and  varied  assemblage  of  fishes  is,  with  the 
exception  of  two  genera,  extinct.  It  is  probably  from  this  order  that 
the  malacopterygian  Teleostei  arose,  since  there  is  a  gradual  transi- 
tion between  the  two  groups.  As  the  name  indicates,  these  fishes  have 
a  completely  ossified  skeleton,  much  like  that  of  the  Teleostei.  The 
scales  are  either  rhombic  or  cycloid  but  are  covered  externally  with 
a  hard  coating  of  ganoin.  If  one  begins  with  the  oldest  fossil  Holos- 
tei  and  proceeds  through  a  series  up  to  more  recent  forms,  it  is  pos- 
sible to  trace  the  development  of  many  of  the  features  that  charac- 
terize the  present  teleosts  and  to  note  the  elimination  of  many  of 
the  more  primitive  teleostome  characters.  The  Holostei  are  repre- 
sented to-day  by  two  families,  the  Lepidosteidce  (Gar-pikes)  and  the 
Amiidce)  Bow-fins.  Only  two  species  of  Gar-pike  and  one  species  of 
Bow-fin  (Amia  calvd)  exist  to-day. 

The  Gar-pikes,  Lepidosteus  (Fig.  72,  A),  are  fresh-water  fishes 
of  the  United  States  and  Canadian  waters.  They  are  elongated  crea- 
tures with  long  slender  snouts  heavily  armed  with  teeth.  They  are 
predaceous  and  kill  great  numbers  of  other  fishes.  In  some  regions  a 
bounty  is  placed  on  their  heads.  The  great  Alligator  Gar  reaches  a 
length  of  ten  feet  or  more  and  with  its  huge  jaws  is  a  really  formidable 


134 


VERTEBRATE  ZOOLOGY 


creature.  The  fins  are  shaped  much  as  in  teleosts,  resembling  those 
of  the  pickerel.  The  scales  are  rhombic  and  heavily  coated  with  ga- 
noin,  so  as  to  make  a  complete  coat  of  mail. 

The  "  Bow-fin,"  Amia  calva  (Fig.  72,  B),  or  "  fresh-water  dog- 
fish," is  quite  like  a  typical  teleost  in  form,  and  would  readily  pass 
for  a  member  of  that  group,  except  for  certain  minor  ganoid  features. 
Amia  is  a  voracious,  carnivorous  fish,  living  in  central  and  southern 
North  America.  Its  most  salient  characters  are :  its  continuous  dor- 


FIG.  72. — Holostei.    A,  Short-nosed  Gar  Pike,  Lepidosleus  platystomus  (after 
Goode).    B,  The  Bow-Fin,  Amia  calva  (after  Bridge). 

sal  fin  (from  which  its  name,  " bow-fin"  is  d^ived) ;  its  heavy  ganoin- 
covered,  imbricating  scales;  its  hompceroal  -(really  modified  hetero- 
cercal)  tail-fin.  It  has  a  cellular  air-bladder  which  it  uses  as  a  lung, 
coming  frequently  to  the  surface  to  gulp  air.  Amia  breeds  in  May 
and  June,  building  a  nest  in  water  weeds,  in  which  it  lays  its  eggs  and 
which  the  male  guards  until  the  eggs  hatch.  Even  after  hatching 
the  young  remain  in  a  school  about  the  male,  who  appears  to  exercise 
some  degree  of  parental  care  over  them.  The  egg  of  Amia  is  a  primi- 
tive one  and  the  cleavage  is  holoblastic  or  nearly  so,  the  whole  de- 
velopment being  more  like  that  of  an  amphibian  than  like  that 
of  a  teleost. 


PISCES 


135 


ORDER  IV.    TELEOSTEI 

The  teleosts  as  a  group  appeared  first  in  Jurassic  times,  evidently 
as  a  derivative  from  the  holostean  ganoids.  Some  of  the  more  primi- 
tive members  of  the  teleostean  sub-order  Malacopterygii,  have  cer- 
tain structures  that  are  reminiscent  of  the  Holostei,  e.  g.,  ganoid 
scales,  multivalvular  conus  arteriosus,  fulcra  in  connection  with  the 
fin  bases,  and  spiral-valve  in  the  intestine.  In  the  higher  teleosts 


FIG.  73. — Showing  the  embryos  of  the  fish  Rhodeus  amarus  living  parasitically 
in  the  gill  cavities  of  the  elam,  Unio.  e,  embryos;  g,  interlamellar  cavities;  i.  l.j, 
an  inter-lamellar  junction.  (From  Bridge,  after  Olt.) 

none  of  these  characters  are  found.  The  teleosts  illustrate  better 
than  almost  any  other  vertebrate  group  the  principles  of  adaptive 
radiation  (Fig.  53).  They  started  modestly  in  the  Jurassic,  increased 
rapidly  through  both  Lower  and  Upper  Cretaceous,  while  in  the  Ter- 
tiary they  had  radiated  adaptively  into  all  of  the  principle  types  of 
body  form  that  characterize  the  modern  condition.  When  we  say 
that  the  teleosts  are  a  modern  group  we  do  not  forget  that  the  group 
originated  many  millions  of  years  ago  and  has  been  a  dominant  group 


136 


VERTEBRATE  ZOOLOGY 


for  at  least  three  or  four  millions  of  years.  The  teleosts  constitute 
now  a  climax  group,  at  the  height  of  its  adaptive  radiation;  a  group 
in  which  specialization  for  strange  and  limited  environments  has  gone 

A 


FIG.  74. — Deep-sea  fishes.  A,  Pholostomias  guemei,  length  1.5  inches  taken  at 
3500  feet;  B,  Idiacanthus  ferox,  8  inches,  16,500  feet;  C,  Gastrostomas  bairdii, 
18  inches,  2300-8800  feet;  D,  Cryplopsarus  couesii,  2.25  inches,  10,000  feet; 
E,  F,  Linophryne  lucifer,  2  inches.  (From  Lull,  after  Goode  and  Bean.) 

to  extremes;  in  which  structures,  especially  integumentary  features 
such  as  spines,  scales,  and  skeletal  elements,  tend  to  run  to  excesses ; 
in  which  pigmentation  and  general  color  characters  develop  into  the 


PISCES  137 

most  elaborate  color  schemes.  Many  teleosts  are  characterized  by 
exaggeration  in  the  development  of  the  fins,  both  median  and  paired; 
others  have  the  snout  and  jaws  over-  or  underdeveloped  for  some 
peculiar  feeding  processes.  Peculiar  breeding  habits  are  accompanied 
by  odd  and  unique  specializations,  like  the  brood-pouches  in  male 
pipe-fishes  and  sea-horses,  and  the  ovipositor  b*y  means  of  which  the 
female  Rhodeus  amarus  deposits  her  eggs  in  the  mantle  cavity  of  the 
clam  Unio,  where  they  develop  safely  and  in  a  well  aerated  environ- 
ment (Fig.  73) .  Perhaps,  however,  the  most  extreme  special  adapta- 
tions are  those  seen  in  deep-sea  fishes  of  a  number  of  teleost  families 
(Fig.  74) .  Two  main  types  are  developed,  the  swift  moving  types  that 
hunt  their  prey  and  the  sluggish  forms  that  lie  in  wait.  These  fishes, 
through  the  use  of  certain  physical  principles  not  yet  fully  under- 
stood, are  able  to  resist  the  pressure  of  thousands  of  tons  of  water  and 
to  maintain  life  at  temperatures  just  above  freezing  and  in  the  practi- 
cally total  absence  of  light.  In  compensation  for  this  life  in  the  dark-- 
ness  they  are  provided  with  a  great  variety  of  phosphorescent  organs 
that  enable  them  to  find  their  way  about. 

THE  TELEOST  SUB-OKDERS 

The  classification  of  the  immense  order  Teleostei  is  a  matter  about 
which  there  is  no  consensus  of  opinion.  The  various  subdivisions 
such  as  the  Acanthopterygii  or  Malacopterygii  are  given  full  ordinal 
value  by  Jordan,  and  merely  subordinal  value  by  Boulenger.  The 
present  writer,  on  the  basis  of  extensive  hybridization  experiments 
with  numerous  species  of  teleosts,  is  inclined  to  believe  that  these  sub- 
orders should  be  given,  at  the  most,  family  value;  for,  the  fact  that  any 
two  species  of  teleost  can  be  crossed  without  artificial  aids  indicates 
that  they  are  fundamentally  not  very  distantly  related.  The  thir- 
teen sub-orders  of  Boulenger  will  be  briefly  surveyed,  especial  empha- 
sis being  given  to  those  groups  that  are  economically  important  or 
phylogenetically  significant.  Certain  types  that  are  either  markedly 
generalized  or  strikingly  specialized  will  receive  attention  to  the  ex- 
clusion of  a  large  number  of  types  interesting  primarily  to  the  spe- 
cialist. 

Primitive  Teleosts  (Malacopterygii). — This  sub-order  contains 
a  very  large  number  of  species  belonging  to  the  Isospondyli  and 
Scyphophori  of  Cope.  About  the  Isospondyli  Jordan  says: — "Of 
the  various  subordinate  groups  of  fishes,  there  can  be  no  ques- 


138  VERTEBRATE  ZOOLOGY 

tion  as  to  which  is  the  most  primitive  in  structure,  or  as  to  which 
stands  nearest  the  orders  of  Ganoids.  Earliest  of  the  bony  fishes 
in  geological  time  is  the  order  Isospondyli,  containing  the  allies, 
recent  or  fossil,  of  the  herring  and  the  trout."  There  are  twenty- 
one  families  of  Malacopterygii  of  which  the  best  known  are  the 
Elopidce  (Tarpons),  the  Salmonidae  (Salmon  and  Trout),  and  the 
Clupeidce  (Herrings). 

The  Tarpon  (Fig.  75)  is  probably  the  noblest  member  of  the 
sub-order  and  is  often  called  the  "Silver  King."     It  is  the  favorite 


FIG.  75. — Tarpon,  Megalops  atlaniicus,  much  reduced.  (From  Boulenger, 
after  Goode.) 

game  fish  along  the  Florida  and  Carolina  coasts;  for  it  is  a  great 
fighter  and  gives  the  sportsman  the  fullest  scope  for  the  ex- 
ercise of  his  skill  and  experience.  It  reaches  a  length  of  six 
feet  and  weighs  over  one  hundred  pounds.  Its  very  large  sil- 
very scales  containing  ganoin  are  used  extensively  in  ornamental 
work. 

The  Salmon  and  Trout  tribes  are  of  all  fishes  the  gamiest  and 
the  most  sought  after  by  the  devotee  of  the  rod  and  fly.  They  are 
characterized  by  the  presence  of  an  adipose  dorsal  fin.  "Of  all  fam- 
ilies of  fishes,"  says  Jordan,  "the  most  interesting  from  every  point 
of  view  is  that  of  the  Salmonidse,  the  salmon  family.  As  now  re- 
stricted, it  is  not  one  of  the  largest  families,  as  it  comprises  less  than 
a  hundred  species;  but  in  beauty,  activity,  gameness,  quality  as  food, 
and  even  in  size  of  individuals,  different  members  of  the  group  stand 
easily  first  among  fishes."  The  Salmon  (Fig.  76)  is  a  marine  fish,  but 
spawns  far  up  among  the  small  streams  near  the  sources  of  large  rivers. 
This  habit  has  given  rise  to  the  "Parent  Stream  Theory,"  according 
to  which  the  young  Salmon  go  down-stream  and  out  to  sea,  where  they 
remain  for  five  years  until  sexually  mature,  and  then  return  to  spawn 


PISCES  139 

in  the  same  parent  stream.  This  does  not  necessarily  imply  any 
marvelous  homing  instinct  or  geographic  sense,  for  it  has  been  found 
that  when  the  Salmon  goes  to  sea  it  does  not  wander  very  far  from  the 
mouth  of  the  particular  river  down  which  it  has  come.  The  instinct 
to  spawn  in  the  smaller  streams  must,  nevertheless,  be  extremely 
impelling,  for  they  frequently  wear  themselves  out  and  die  owing  to 
the  arduous  up-stream  journey  of  often  more  than  a  thousand  miles, 
through  rapids  and  even  over  water-falls  of  considerable  height.  It 


o 


FIG.  76. — Salmon,  Salmo  fario.  a.  I,  adipose  lobe  of  pelvic  fin;  an,  anus;  c.f, 
caudal  fin;  d.f.  1,  first  dorsal  fin;  d.f.  2,  second  dorsal  or  adipose  fin;  1. 1,  lateral 
line;  op,  operculum;  pct.f,  pectoral  fin;  pv.f,  pelvic  fin;  v.f.  ventral  fin.  (From 
Parker  and  Haswell,  after  Jardine.) 

is  said  that  few,  if  any,  survive  to  go  down-stream  and  out  to  sea 
again;  a  statement  that  seems  to  be  out  of  accord  with  the  fact  that 
some  very  large  specimens,  evidently  much  over  five  years  old,  are 
captured  in  every  salmon  river. 

The  great  Herring  family  (Clupeidce)  consists  of  fishes  of  de  - 
cidedly  generalized  proportions  and  characters.  They  are  diagram- 
matic teleosts.  Fossil  Herrings  practically  like  those  of  the  present 
have  been  found  well  preserved  in  Cretaceous  rocks.  The  family 
includes  also  Shad,  Anchovies,  and  White-fishes,  which  are  among  the 
most  important  of  the  world's  food  fishes. 

By  no  means  all  of  the  Malacopterygii  are  generalized  types,  for 
there  has  been  a  very  considerable  adaptive  radiation  within  the 
group.  Among  the  specialized  and  senescent  types  are:  the  mormy- 
rids  of  the  Nile,  remarkable  electric  fishes  that  were  pictured  by  the 
early  Egyptians;  eel-like  types  such  as  Gymnarchus;  proboscis- 
fishes  such  as  Gnaihonemus;  and  several  deep-sea  forms  with  degen- 
erate and  otherwise  aberrant  characters. 


140  VERTEBRATE  ZOOLOGY 

The  Cat-fishes,  Carps  and  Their  Kin  (Ostariophysi) . — This  is  one 
of  the  most  clearly  defined  of  the  sub-orders,  and  one  that  possesses 
certain  primitive  characters  that  suggest  ganoid  affinities.  The 
Cat-fishes  (Siluridce)  are  very  common  fishes  of  cosmopolitan  dis- 
tribution, distinguished  by  the  presence  of  spines  and  barbels  about 
the  mouth  (Fig.  77).  They  are  as  a  rule  sluggish  and  mud-loving. 
Some  of  the  Cat-fishes  reach  a  very  large  size,  growing  to  be  ten  feet 


FIG.  77. — A  siluroid  fish,  Rita  buchanani.  b,  barbel;  d.  f.r.  1,  first  dorsal  fin-ray; 
d.f.  2,  adipose  fin;  pct.f.  r.  1,  first  pectoral  fin-ray;  pv.f,  pelvic  fin;  y./,  ventral  fin. 
(From  Parker  and  Haswell,  after  Day.) 

long  and  weighing  in  the  neighborhood  of  four  hundred  pounds.  The 
Gymnotidce  are  the  Electric  Eels  of  South  America,  the  best  known 
species  being  Gymnotus  electricus,  a  large  eel-like  fish  about  eight 
feet  in  length,  and  much  feared  by  the  natives  on  account  of  the  sever- 
ity of  shock  it  is  capable  of  delivering.  The  Characinidce  comprise 
about  five  hundred  species  of  African  and  South  American  fishes,  that 
on  the  whole  are  the  most  generalized  representatives  of  the  sub-order; 
their  most  highly  specialized  feature  is  their  rather  elaborate  denti- 
tion, which  is  associated  with  their  carnivorous  habits.  The  Cyprini- 
dce  (Carps)  are  also  very  generalized,  so  much  so  that  some  authori- 
ties place  them  at  the  bottom  of  the  scale  of  modern  teleosts.  A 
number  of  senescent  armored  types  of  Ostariophysi  are  known,  most 
of  which  live  in  unusual  habitats. 

The  Symbranchii. — This  small  sub-order  of  eel-like  fishes  is  doubt- 
less a  senescent  derivative  of  either  the  Ostariophysi  or  the  Apodes. 
They  are  without  paired  fins,  have  the  gill  openings  united  into  a 
single  ventral  slit,  and  have  no  air-bladder.  For  our  purposes  no 
further  characterization  is  necessary. 

The  Eels  and  Their  Kin   (Apodes). — This  sub-order  comprises  a 


PISCES  141 

very  large  number  of  eel-like  fishes  which  are  in  all  probability 
polyphyletic  in  origin  and  have  been  placed  in  the  same  group 
on  account  of  possessing  in  common  the  attributes  generally  asso- 
ciated with  eels.  The  present  writer  would  hazard  the  suggestion 
that  they  may  be  degenerate  derivatives  of  several  groups  of 
fishes  that  have  undergone  the  same  type  of  racial  degeneration; 
thus  the  Apodes  present  a  situation  comparable  with  that  of  the  per- 
ennibranchiate  Amphibia,  a  group  now  adjudged  to  be  of  polyphyletic 
origin  and  psedogenetic  in  character.  It  seems  not  unlikely  that  the 
eels  are  the  victims  of  racial  retardation,  due  to  some  kind  of  develop- 
mental defect  that  inhibits  the  normal  differentiation  of  the  anterior 
parts  of  the  primary  axis  and  that  of  the  bilateral  appendages.  Some 


FIG.  78. — Eel  or  Moray,  Gymnoihrax  waialuce,  from  Hawaii  (after  Jordan  and 
Evermann.) 

of  the  eels  show  still  more  radical  distortions  of  the  generalized  fish 
proportions  in  having  excessively  elongated  bodies,  as  is  the  case  in  the 
Thread-eel  (Nematichthys),  which  one  can  hardly  believe  to  be  a  fish 
at  all.  Almost  as  remarkable  are  the  Gulper-eels  (Fig.  74,  C),  abysmal 
forms  with  enormous  head  and  mouth  and  a  much  attenuated  body 
ending  in  a  filamentous  tail.  Of  these  Dr.  Gill  says: — "The  entire 
organization  is  peculiar  to  the  extent  of  anomaly,  and  our  old  con- 
ceptions of  a  fish  require  to  be  modified  in  the  light  of  our  knowledge  of 
such  strange  beings."  The  Morays,  a  great  family  of  marine  eels,  are 
predaceous  fishes  of  great  efficiency,  with  highly  developed  teeth  and 
often  with  color  patterns  strikingly  elaborate  and  brilliant.  These 
patterns  often  simulate  those  of  snakes,  as  in  the  banded  species, 
Gymnothorax  (Fig.  78);  it  is  said  that  they  are  often  quite 
poisonous. 


142 


VERTEBRATE  ZOOLOGY 


Pikes  and  Killifishes  (Haplomi). — The  Pikes  (Esocidse)  are  well- 
known  fresh-water  fishes  with  long  body  and  large  mouth.  The 
finest  of  the  Pikes  is  the  Muskalunge  (Esox  masquinongy) ,  the 
largest  of  our  inland  game  fishes.  It  reaches  a  length  of  six  feet  and 
a  weight  of  as  high  as  eighty  pounds;  but  the  fisherman  that  gets  a 
good  strike  from  a  twenty-five  pounder  has  about  as  much  as  he 
can  handle. 

Fundulus  heteroditus  (Fig.  79),  the  common  Killifish  or  Mud- 
minnow,  is  perhaps  worthy  of  special  mention  on  account  of  its 


B 


FIG.  79. — Fundulus  heteroditus ,  the  Killifish.     A,  female,  B,  male;  showing 
sexual  dimorphism  in  fins,  and  in  markings.    (From  Newman.) 

important  contribution  to  experimental  biology.  The  eggs  of  this 
species  have  furnished  more  material  for  investigation  than  those  of 
any  other  fish.  If  the  literature  on  F.  heieroclitus  were  collated  it 
would  constitute  many  large  volumes.  The  family  (Cyprinodon- 
tidce  or  Pceciliidce)  to  which  Fundulus  belongs,  contains  several  vivip- 
arous species,  in  which  the  anal  fin  of  the  male  is  used  as  an 
intromittent  organ  for  introducing  sperm  into  the  oviducts  of  the 
female. 

The  Heteromi.— This  comparatively  small  order  of  degenerate  or 
senescent  fishes  may  be  passed  over  with  little  comment.    Most  of 


PISCES 


143 


the  species  are  deep-sea  forms.  One  genus,  Fierasfer  (Fig.  80),  is 
remarkable  on  account  of  its  commensal  life  as  a  lodger  in  the  vari- 
ous cavities  of  echinoderms  and  mollusks.  One  species,  according  to 
Boulenger,  enters  its  host,  a  holothurian,  through  the  anal  opening, 
and  lies  with  the  head  only  protruding.  From  time  to  time  it  dashes 
out  after  its  prey  and  returns  to  its  shelter  to  eat  it.  This  could 


FIG.  80. — Fierasfer  acvs,  penetrating  the  anal  openings  of  holothurians,  2/i 
natural  size.  (From  Boulenger,  after  Emery.) 

hardly  be  interpreted  as  a  case  of  symbiosis,    for  there  can  be  no 
mutuality  in  the  arrangement. 

Stickle-backs,  Pipe-fishes,  and  Sea-horses  (Catosteomi) .'—  This 
well-defined  group  of  peculiar  fishes  exhibits  a  wide  range  of  spe- 
cialization and  senescence.  The  Stickle-backs  themselves  (Fig.  81), 
apart  from  their  side-armor  and  prominent  spines,  are  quite  general- 
ized in  their  proportions.  Nothing  less  fish-like,  however,  could  well 
be  imagined  than  some  of  the  extreme  Sea-horses,  which  look  more 
like  gargoyles  than  real  animals.  The  typical  Sea-horse  (Fig.  82) 
might  be  compared  with  a  knight  of  a  set  of  chessmen,  with  a  long, 
coiled  tail  instead  of  a  base.  •  The  Pipe-fishes  may  be  considered  as  the 


144 


VERTEBRATE  ZOOLOGY 


"eels"  of  the  Catosteomi,  for  they  are  much  like  greatly  attenuated 
Sea-horses. 

The  breeding  habits  of  all  members  of  the  sub-order  are  peculiar. 


FIG.  81. — Gasterosteus  aculeatus.    xl.    (From  Boulenger,  after  Goode.) 

In  the  case  of  the  Stickle-backs  the  male  builds  a  nest  out  of  green 
grasses  and  kindred  materials,  leaving  a  front  and  a  rear  entrance. 
When  the  nest  is  complete  he  goes  a-wooing  and 
induces  a  female  to  enter  his  nest  and  lay  her 
eggs  there.  As  soon  as  she  leaves  by  the  back 
door  he  enters  by  the  front  and  fertilizes  the 
eggs.  Usually  several  other  females  are  em- 
ployed in  the  same  way  until  the  nest  is  filled 
with  a  sticky  mass  of  eggs.  He  then  watches 
over  the  nest  until  the  eggs  are  all  hatched.  Sea- 
horses carry  to  a  higher  degree  of  specialization 
this  paternal  solicitude  for  the  welfare  of  off- 
spring, for,  instead  of  building  a  nest  and  guard- 
ing the  eggs,  the  male  uses  a  part  of  his  body, 
a  brood-pouch  on  the  abdomen,  as  a  nest.  Ac- 
cording to  Jordan,  the  female  lays  her  eggs  on 
FIG.  82.  —  Hippo-  the  sea-bottom,  and  the  male,  after  inseminat- 

i£d£!UC£££  ing  them>  transfers  them  to  the  brood-p°uch  and 

pouch  (mp).  a,  anus;  carries  them  about  until  they  are  hatched,  thus 
b.  a,  branchial  aper-  making  of  himself  an  animated  incubator. 
"  Some  of  the  Sea-horses  are  provided  with  an 
elaborate  camouflage  in  the  form  of  leaf-like 
processes  (Fig.  83)  colored  like  sea-weed  and  are  practically  invis- 
able  in  their  native  haunts.  The  Sea-moth,  another  member  of  the 
Catosteomi,  is  almost  as  fantastic  as  the  Sea-horses.  It  is  covered 


PISCES 


145 


with  heavy  armor  and  has  enormous  pectoral  fins  that  give  it  a 
moth-like  aspect. 

The  Flying-fishes  and  Their 
Allies  (Percesoces) . — While  flying- 
fish  types  are  found  in  several 
other  sub-orders  of  fishes,  those 
of  this  group  are  perhaps  the  most 
highly  specialized.  One  of  the 
best  types  is  Exonautes  (Fig.  84), 
which,  when  it  leaps  out  of  the 
water,  parachutes  for  some  distance 
by  means  of  its  very  large  pec- 
toral fins. 

The  sub-order  is  not  believed 
to  be  homogeneous,  for  it  con- 
tains such  aberrant  forms  as  Be- 
lone,  a  form  resembling  superficially 
the  Gar-Pike,  and  sometimes  given 
that  name. 

The  Cod  and  Their  Kin  (Ana- 
canihini). — Apart  from  the  Cod- 
fishes the  members  of  this  group 

are   rather   unfamiliar  and   of  no      r-     QQ     m,  //  ^/ 

FiG.SS.—Phyllopteryxeques^Anat- 

especial   interest  for  us.    The  Cod   ural  size.    (After  Boulenger.) 

(Fig.  85),  however,  is  one  of  the 

most  important  of  the  world's  food  fishes.     It  is  one  of  the  most 


FIG.  84. — Flying  fish,  Exonautes  gilberti.     (After  Jordan  and  Evermann.) 


146  VERTEBRATE  ZOOLOGY 

voracious  and  omnivorous  of  fishes.  It  breeds  far  out  at  sea,  and 
its  tiny  pelagic  eggs  are  almost  inconceivably  numerous;  as  many  as 
nine  million  eggs  have  been  estimated  as  being  laid  by  a  single  large 


y» 


FIG.  85. — Gadus  mmrhua  (Cod),  an,  anus;  cf,  caudal  fin;  dfl-3,  dorsal  fins; 
mx,  maxilla;  pct.f,  pectoral  fin;  pmx,  premaxilla;  pv.f,  pelvic  fin;  vf.  1  and  2, 
ventral  fins.  (From  Parker  and  Haswell,  after  Cuvier.) 

female  in  one  season.  Most  of  us  who  have  been  children  need 
hardly  be  reminded  of  the  somewhat  unpleasant  fact  that  the 
liver  of  the  Cod  is  the  source  of  a  highly  nutritious  and  readily 
digested  oil.  Relatives  of  the  Cod  are  the  Haddock,  the  Pollock, 
the  Burbot,  the  Hake,  and  some  aberrant  and  degenerate  deep- 
sea  forms. 

The  Spiny-Rayed  Fishes  (Acanthopterygii) . — This  tremendous 
assemblage  of  modernized  fishes  reminds  one  of  the  passerine 
birds,  because  they  are  the  most  modern  of  the  sub-orders  and 
show  a  more  extensive  adaptive  radiation  than  do  any  of  the 
other  groups.  The  Acanthopterygii  comprise  no  less  than  thirty- 
six  families  including  such  familiar  forms  as  the  Bass,  Perch,  Flounder, 
Goby,  etc.,  and  a  host  of  less  familiar  types. 

The  common  Perch  is  as  good  a  type  to  illustrate  the  sub-order  as 
any,  though  it  is  perhaps  the  most  generalized  member  of  the  group. 
Many  of  the  others  tend  to  become  high,  compressed,  and  short- 
bodied,  such  as  the  little  fresh-water  sun-fish.  Other  forms  that  live 
in  the  open  seas  have  carried  out  this  line  of  development  till  the 
dorso-ventral  axis  appears  to  overshadow  the  primary  and  the  bi- 
lateral axes,  as  is  the  case  in  some  of  the  Acanthiuridce,  a  family 
which  is  strikingly  exemplified  by  Zanclus  (Fig.  86) .  It  is  from  such 
types  as  this  that  the  members  of  the  curious  sub-order  Pledognaihi 
are  thought  to  have  been  derived 


PISCES 


147 


The  Mackerel  family  (Scombridce)  is  one  of  the  most  generalized 
members  of  the  sub-order  and  one  of  the  most  important  as  food 
fishes. 


FIG.  86. — Zanclus  canescens,  Linn.    (Redrawn  after  Jordan  and  Evermann.) 

The  Flounders  or  Flat-fishes  (Pleuronectidce)  are  among  the 
most  aberrant  of  all  fishes  (Fig.  87).  So  unique  are  they  in  their 
peculiarities  that  Jordan  sees  fit  to  place  them  in  a  separate  sub-order, 


FIG.  87. — Pleuronectes  cynoglossus  (Flounder),  from  the  right  side,  d.f,  dorsal 
fin;  1.  e,  left  eye;  r.  e,  right  eye;  pet.  /,  pectoral  fin;  pv.  /,  pelvic  fin.  (From  Parker 
and  Haswell,  after  Cuvier.) 


148 


VERTEBRATE  ZOOLOGY 


which  he  calls  Heterosomata.  The  Flounders  are  bottom-fishes  that 
lie  on  the  side  instead  of  on  the  belly  as  do  most  other  bottom-fishes; 
some  species  lie  on  the  right,  others  on  the  left  side.  To  adapt  them- 
selves to  this  position  there  is  a  remarkable  twisting  of  the  cranium 
that  results  in  bringing  both  eyes  on  the  same  side  of  the  head,  and 
makes  the  whole  region  of  the  head  decidedly  asymmetrical.  The 
upper  side  of  the  body  becomes  variously  pigmented  in  harmony 
with  almost  any  background;  experiments,  involving  the  use  of  the 


FIG.  88. — The  Shark  Sucker,  Remora  brachyptera,  Lowe  (above).    (From  Jordan 
and  Evermann.) 

FIG.  89. — Sucking  disc  of  Remora  brachyptera,  Lowe  (below).     Dorsal  view. 
(From  Jordan  and  Evermann.) 

most  elaborate  of  artificial  backgrounds,  having  proven  their  ex- 
traordinary capacity  for  imitation.  The  lower  side  normally  remains 
unpigmented,  but,  if  it  is  artificially  illuminated  by  growing  the  fishes 
on  an  elevated  glass  floor  in  an  aquarium,  it  acquires  an  appearance 
much  like  the  upper  side.  The  young  flounder  is  bilaterally  symmet- 
rical and  begins  the  head-twisting  process  some  time  before  it  takes 
up  the  bottom-living  habit.  Flounders  are  believed  to  have  been  de- 
rived from  some  one  of  the  high  compressed  types,  which  adopted  the 
bottom-feeding  habit  and  was  forced  to  modify  itself  in  a  peculiar 
way  to  meet  the  new  conditions. 


PISCES 


149 


The  Gobies  (Gobiidce)  are  surface-dwelling  fishes  characterized 
by  elaborate  fin  structures  and  brilliant  colors,  reminding  one  of 
some  of  the  brilliant  birds.  They  might  be  designated  the  "humming- 
birds" among  fishes.  The  Gobies  are  excessively  numerous  in  tropi- 
cal waters.  The  East  Indian  Goby  has  large  muscular  pectoral  fins, 
which  it  uses  like  feet;  its  habit  is  to  hop  about  over  the  mud-flats  at 
low  tide,  feeding  upon  stranded  crustaceans. 

The  Shark-suckers  (Remoras)  are  especially  noteworthy  on 
account  of  the  peculiar  modifications  of  the  anterior  dorsal  fin  into  a 
lamellated  sucker  (Figs.  88  and  89),  by  means  of  which  they  adhere  to 
the  body  of  a  shark  or  some  other  smooth-bodied  fish.  By  means  of 
this  sucking  disk  they  obtain  free  transportation  without  exertion, 


FIG.  90. — Type  of  teleost  with  over-specialized  fins. 
(Redrawn  after  Jordan  and  Evermann.) 


Dendrochirus  hudsoni. 


dropping  off  when  they  reach  a  desired  destination  and  awaiting 
another  accommodating  conveyance  when  they  wish  again  to  travel. 
They  appear  to  do  no  harm  to  the  host  fish,  except  in  so  far  as  they 
somewhat  impede  its  movements. 

The  family  Scorpcenidce  (Mailed-Cheeked  Fishes)  are  among  the 
most  elaborately  finned  of  the  Spiny-Rayed  fishes.  A  good  type  is 
Dendrochirus  (Fig.  90),  in  which  the  fins,  both  paired  and  median, 
appear  to  have  run  riot  like  the  plumage  of  some  of  the  highly  spe- 
cialized birds.  The  color  pattern  is  in  striking  accord  with  the  back- 
ground. 


150  VERTEBRATE  ZOOLOGY 

A  number  of  highly  specialized  and  exaggerated  types  of  Spiny 
Rayed  Fishes  might  be  mentioned.  The  Deal-Fish  or  Ribbon-Fish 
is  an  extremely  elongated  and  laterally  compressed  type,  that  is  said 
to  hold  one  side  obliquely  upward  when  swimming.  Some  of  these 
fishes  attain  a  length  of  twenty  feet.  There  are  also  several  types  of 
degenerate  abysmal  species,  both  of  the  eel-type  and  the  head-fish 
types.  Some  of  the  best  types  of  flying-fishes,  notably  the  "  Flying 
Gurnards  "  belong  to  this  sub-order,  fishes,  which,  when  they  leap  from 
the  water,  flutter  their  large  pectoral  fins  like  a  flying  grasshopper. 
One  of  the  most  extreme  specializations  is  seen  in  the  case  of  Anabas, 
the  Climbing  Perch  (Fig.  91),  a  species  in  which  there  is  a  superbranch- 


FIQ.  91. — Anabas  scandens  (Climbing  Perch).  (After  Parker  and  Haswell.) 

ial  lung-like  organ,  by  the  use  of  which  the  fish  is  enabled  to  live  for 
long  periods  out  of  water.  It  climbs  up  low  trees  using  the  spines  on 
its  gill-covers  and  pectoral  fins  like  claws.  It  is  thus  able  to  capture 
insect  and  other  food  that  would  be  unavailable  for  a  strictly  aquatic 
fish.  The  Toad-fishes  are  rather  sluggish,  large-headed,  wide-mouthed 
forms  that  are  especially  noteworthy  for  their  extreme  ugliness.  It 
is  believed  that  they  represent  the  ancestral  group  from  which  the 
sub-order  Pediculati  (Anglers)  has  been  derived. 

The  Opisthomi. — This  is  a  small  sub-order  of  degenerate  eel-like 
fishes,  believed  to  have  been  derived  from  the  Acanthopterygii.  For 
our  purposes  they  may  be  passed  without  further  comment. 

The  Anglers  (Pediculati). — This  sub-order  of  highly  specialized, 
senescent  fishes  is  believed  to  have  been  derived  from  the 
Toad-Fishes,  an  aberrant  family  of  Acanthopterygii.  They  seem 


PISCES  151 

to  be  little  more  than  a  head  with  an  enormous  gaping  mouth 
(Fig.  74,  E  and  F).  The  anterior  spine  of  the  dorsal  fin  is  modi- 
fied so  as  to  form  a  sort  of  "fishing-rod"  which  hangs  over  the 
mouth  and  has  pendant  from  its  tip  a  fleshy  pad  or  bait.  The 
Anglers  are  almost  scaleless  and  are  bottom-feeders,  both  of  which 
characters  are  taken  to  be  evidences  of  senescence.  Some  of  them  are 
abysmal  forms  that  have  been  called  "Bathymal  Sea  Devils";  these 
are  even  more  degraded  in  structure  than  are  the  more  typical  An- 
glers. One  of  the  strangest  of  all  fishes  is  the  Sargassum  Fish,  a  form 
that  lives  a  drifting  life  among  the  masses  of  Sargassum  weed;  its 
camouflage  of  color  pattern  and  raggedly  weed-like  fins  make  it  merely 
a  part  of  the  general  floating  mass.  The  Bat-Fish  represents  perhaps 
the  climax  of  senescent  degeneration  in  this  group;  it  is  a  very  broad, 
flat  bottom-feeder,  with  wing-like  pectoral  fins. 

Fool-Fishes,     Trunk-Fishes,     Porcupine-Fishes,    Puffers,    and 
Head-Fishes    (Plectognathi) . — This    final    sub-order  of    teleosts    is 


FIG.  92. — Hawaiian  Trigger-Fish,  Ballistapus  rectangulus.     (Redrawn  after 
Jordan  and  Evermann.) 

perhaps  the  most  highly  specialized  of  all,  and  is  believed  to  be 
derived  from  the  family  Acanthiuridae  of  the  Acanthopterygii,  which 
are  characteristically  short,  high,  compressed  fishes,  typified  by 
Zanclus  (Fig.  86).  The  Plectognathi  comprises  a  collection  of  the 
strangest  creatures  that  the  sea  affords.  Of  these  the  Trigger- 
Fishes  are  the  most  moderate  in  structure,  not  unlike  some  of 
the  Acanthopterygii  in  their  high,  compressed  proportions;  a  modi- 


152 


VERTEBRATE  ZOOLOGY 


fied  spine  of  the  dorsal  fin  on  top  of  the  head  looks  like  a  trigger 
and  gives  them  their  name.  The  File-Fishes  or  Fool-Fishes  (Fig.  92) 
are  still  more  flatly  compressed  and  the  scales  are  reduced  to  vesti- 
gial structures;  the  small  mouth  with  its  protruding  teeth  and  the 
funny  staring  expression  of  the  eyes  have  given  them  an  uncompli- 
mentary name.  The  Trunk-Fishes  (Fig.  93)  are  big-headed  fishes  in- 
closed in  a  heavy  immovable  armor,  composed  of  closely  united 
plates,  with  a  large  posterior  opening  that  allows  the  curious  little 
tail  to  waggle,  and  smaller  openings  for  the  pectoral,  dorsal,  and  anal 
fins.  Puffers  or  Globe-Fishes  are  unarmored  forms,  shaped,  when  de- 


FIG.  93.— Hawaiian  Trunk-Fish,  Ostrachion  schlemmeri.    (Redrawn  after  Jordan 
and  Evermann.) 

flated,  much  like  Trunk-Fishes,  but  capable  of  blowing  themselves  up 
with  water  to  several  times  their  normal  dimensions,  thus  making 
themselves  difficult  to  swallow.  If  taken  out  of  water  these  strange 
little  fellows  suck  in  air  till  they  are  of  a  drum-like  tightness.  Some  of 
the  Globe-Fishes  are  said  to  be  extremely  poisonous.  The  Porcupine- 
Fishes  are  shaped  much  like  the  Puffers  in  a  deflated  or  partly  de- 
flated condition,  some  being  much  rounder  than  others;  but  they  are 
covered  with  a  heavy  spiky  armor  that  has  suggested  their  name. 
They  also  have  the  reputation  of  being  decidedly  poisonous.  The 
Head-Fishes  or  Sun-Fishes  (Fig.  94)  represent  the  climax  of  relative 
increase  of  head  over  body,  a  character  exhibited  by  the  whole  group; 
they  are  little  more  than  animated  fish  heads.  The  body  is  so  abbre- 
viated that  the  dorsal  and  anal  fins  appear  to  be  attached  to  the  upper 
and  lower  parts  of  the  head.  They  inhabit  the  tropical  and  sub- tropi- 
cal seas,  living  a  sluggish,  floating  life  that  is  almost  sedentary.  Large 


PISCES  153 

specimens  reach  a  giant  size,  being  about  eight  feet  in  diameter  and 
weighing  as  much  as  twelve  hundred  pounds.  The  skeleton  is  largely 
cartilaginous  and  there  is  a  very  heavy  dermal  cartilaginous  armor. 
The  skin  is  smooth  and  scaleless.  All  of  these  characters  will  readily 
be  recognized  as  criteria  of  senescence.  Indeed  it  is  a  question  as  to 


FIG.  94. — Hawaiian  Head-Fish,  Ranzania  makua,  Jenkins.     (Redrawn  after 
Jordan  and  Evermann.) 

whether  these  creatures  or  the  most  degraded  of  the  Pediculati  are 
the  most  senescent  members  of  the  entire  class  of  Fishes.  The  Ped- 
iculati appear  to  be  the  logical  culmination  of  the  rather  broad,  flat 
type  of  head-fish  architecture,  while  the  Sun-Fishes  have  carried  out 
to  the  extreme  the  evolution  of  the  high,  compressed  head-fish  type. 
Both  of  these  tendencies  appear  to  have  been  manifest  in  the  groups 
of  Acanthopterygii  from  which  these  two  sub-orders  have  respectively 
been  derived. 


154  V   /  VERTEBRATE  ZOOLOGY 


SUB-CLASS  III.    DIPNEUSTI   (Dipnoi).    The  Lung-Fishes 

This  group  of  fishes  has  acquired  an  unusual  interest  because  of  the 
belief,  rather  general  until  recent  years,  that  from  it  the  Amphibia 
took  their  rise.  A  number  of  writers  still  appear  to  hold  this  view, 
or  at  least  to  maintain  that  a  close  affinity  exists  between  the  Lung- 
fishes  and  the  Amphibia.  Goodrich,  for  example,  in  the  volume  on 
Cyclostomes  and  Fishes  in  Lankester's  Treatise  on  Zoology,  says: — 
"The  Dipnoi  are  among  the  most  interesting  of  fish.  On  the  one 
hand,  they  have  a  close  affinit  to  the  Osteolepidoti  (extinct 
Crossopterygii) ;  on  the  other,  they  present  many  striking  points 
of  resemblance  to  the  Amphibia,  which  cannot  all  be  put  down 
to  convergence.' 

The  group  is  distinguished  by  the  following  characters : — the  paired 
fins  are  rather  slender  and  pointed;  the  scales  are  cycloid  in  form 
and  overlapping;  the  caudal  fin  is  diphy cereal;  the  upper  jaw  is  firmly 
fused  with  the  base  of  the  skull,  making  a  holostylic  skull;  teeth  are 
largely  lacking,  and  their  place  is  taken  by  large  tritoral  dental  plates, 
supported  by  the  palato-pterygoid  and  splenial  bones;  the  premax- 
illary  and  maxillary  bones  are  absent,  and  the  dentaries,  usually 
absent,  are  vestigial  when  present.  It  would  appear  from  this  sum- 
mary of  characters  that  the  Dipneusti  show  more  evidences  of  being 
a  degenerate  and  senescent  group  than  one  likely  to  give  rise  to  a  new 
and  successful  class  like  the  Amphibia.  Most  of  the  characteristics 
of  these  fishes  lead  away  from  rather  than  toward  amphibian  condi- 
tions. 

The  question  arises  as  to  whether  the  characters  mentioned  were 
more  or  less  primitive  in  extinct  than  in  modern  lung-fishes. 
The  modern  Dipneusti  appear  to  have  retained  certain  primi- 
tive characters,  such  as  the  diphycercal  tail  fin  and  the  entirely 
cartilaginous  condition  of  the  primitive  cranium.  An  examination 
however  of  the  earliest  fossil  Dipneusti,  in  which  the  tail  is  heterocer- 
cal,  the  median  fins  broken  up  into  isolated  dorsals  and  ventrals,  the 
chondrocranium  extensively  ossified,  and  scales  large  and  close  to  the 
surface,  show  that  the  modern  Dipneusti  are  degenerate  and  not 
truly  primitive.  Additional  degeneration  is  seen  in  the  loss  of  maxil- 
lary, premaxillary,  and  dentary  bones,  which  are  present  in  Amphibia. 
The  paired  fins  are  also  almost  vestigial  in  some  modern  lung-fishes 
and  are  so  constructed  as  to  make  the  derivation  from  them  of  any- 


PISCES  . 


155 


thing  like  an  Amphibian  limb  impossible.  Certain  specializations 
are  also  present,  such  as  the  fusion  of  teeth  into  massive  grinding  or 
tritoral  plates,  the  development  of  respiratory  filaments  on  the  pelvic 


^^^& 


FIG.  95.— Group  of  Lung-Fishes  (Dipneusti).  A,  Neoceratodusforsleri,  Queens- 
land. B,  Protopterus  arwectans,  Gambia.  C,  Lepidosiren  paradoxa,  Paraguay. 
(The  lozenge-shaped  markings  in  B  do  not  represent  scales,  but  areas  of  skin 
outlined  by  pigment  cells.  In  a  fresh  specimen  the  scales  are  completely  invis- 
ible, as  in  C.)  D,  Diagram  of  Pioiopleius  aestivating  in  the  mud,  showing  the 
body  coiled  up  and  the  mucous  sac  with  tube  leading  to  mouth.  E,  Larva  of 
Protopterus  on  the  seventh  day,  showing  cutaneous  gills,  cement  organ  under 
head,  and  narrow  paired  fins.  F,  Larva  of  Lepidosiren  thirty  days  after  hatching, 
showing  some  characters  as  E.  (Redrawn  from  Bridge,  A,  after  Giinther;  B,  and 
C,  after  Lankester;  D,  after  Parker;  E,  after  Budgett,  and  F,  after  Kerr.) 


156  \y  VERTEBRATE   ZOOLOGY 

limb  of  Lepidosiren,  the  mud-dwelling  habit,  and  the  highly  special- 
ized air-bladder,  used  as  a  lung. 

The  oldest  representative  of  the  Dipneusti  is  the  genus  Dipterus 
which  occurs  along  with  the  earliest  known  Crossopterygii  and  the 
most  primitive  Actinopterygii  in  Devonian  times.  A  comparison 
between  Dipterus  and  the  early  "lobe  fins,"  such  as  Osteolepis,  shows 
obvious  resemblances.  Dipterus  has  acutely  lobate  paired  fins,  and 
the  skull  bones  are  typically  dipnoan  in  their  lack  of  premaxillary, 
maxillary,  and  dentary,  and  the  presence  of  tritoral  plate  instead  of 
teeth. 

The  larval  stages  of  modern  dipnoans  are  remarkably  like  those 
of  modern  Amphibia.  Those  of  Protopterus  and  Lepidosiren  are 
veritable  tadpoles  (Fig.  95,  E  and  F)  with  external  gills,  tails  with 
unsupported  median  fins,  blunt  heads,  and  clinging  habits.  Just  as  in 
amphibian  larvae,  they  have  a  sucker  or  ventral  cement  organ  for  adhe- 
sion. Evidently  this  tadpole  larva  represents  an  extremely  ancient 
developmental  stage  present  in  the  common  ancestors  of  these  two 
groups  and  retained  with  few  modifications  by  the  modern  classes. 
Indeed  it  seems  certain  that  the  whole  early  development  of  Amphi- 
bia represents  a  more  primitive  evolutionary  stage  than  does  that  of 
any  modern  fish.  It  is  customary  to  deal  with  amphibian  embryology 
as  a  step  in  advance  of  Amphioxus  embryology  and  to  derive  the  va- 
rious types  of  fish  development  from  some  condition  like  that  of  the 
amphibian.  It  is  of  some  interest  to  know,  therefore,  that  the  dip- 
noan fishes  have  a  more  primitive  embryology  (Fig.  96)  than  any 
other  living  fishes,  that  the  bony  ganoids  are  next  in  primitiveness, 
and  that  elasmobranchs,  the  more  primitive  ganoids,  and  the  teleosts 
show  a  specialized  development,  in  some  respects  paralleling  that 
seen  in  the  reptiles  and  birds. 

_JlABiTS  OF  MODERN  DIPNEUSTI 

The  existing  genera  of  dipnoans  are  N^oceraMus  of  Australia, 
Protopterus  of  Africa,  and  Lepidosiren  pi  Paraguay.  They  are  all 
fished  of  stagnant  rivers,  and  fresh-waj;er  pools.  Neoceratodus  is  the 
most  primitive  arid^  saows  less  degeneration;  Ezpidosiren  shows  the 
extreme  reduction  in  fins  and  scales  characteristic  of  the  eel-like  type; 
while  Protopterus  is  just  about  intermediate  between  the  other  two. 
Bridge  has  given  good  accounts  of  the  lives  of  these  singularly  inter- 
esting fishes. 


PISCES  157 

A  socemtodus  forsteri,  a  fish  that  reaches  a  length  of  over  five 
feet  (Fig.  95  A),  "frequents  the  comparatively  stagnant  pools  / 
or  water-holes  which  alternate  with  shallow  runs  and  are  usually 
full  of  water  all  the  year  round.  In  these  pools,  filled  with  a  rich 
growth  t. f  vegetation,  and  often  the  favorite  haunt  of  the  Platy- 
pus (Orni>  ^orhynchus)  the  Fish  is  fairly  abundant.  Inactive  and 
sluggish  in  its  habits,  usually  lying  motionless  on  the  bottom,  the 
Fish  Is^eUsily  captured  by  the  natives  with  hand  nets  and  baited 
hooks.  Neoceratodus :  lives  on  fresh-water  Crustaceans,  worms, 
and  molluscs,  and  to  obtj:^  them  it  crops  the  luxuriant  voirota- 
tion  much  in  the  same  way  that  a  Polychaet  or  a  Holothurian 
swallows  sand  for  the  sake  of  the  included  nutrient  particles. 
Apparent, lyjjie  air-bladder  is  aTTunctional  lunp:  at  all  time^  act- 
ing in  conjunction  with  the  gills. ,  At  irregular  intervals  the  Fish 
rises  to  the  surface  and  protrudes  its  snout  in  order  to  empty  its 
lung  and  take  in  fresh  air.  While  doing  so  the  animal  makes  a 
peculiar  grunting  noise,  '  spouting'  as  the  local  fishermen  call  it, 
which  may  be  heard  at  night  for  some  distance,  and  is  probably 
caused  by  the  forcible  expulsion  of  air  through  the  mouth.  Useful 
as  the  lung  is  as  a  breathing  organ  under  normal  conditions,  there 
can  be  little  doubt  that  its  value  as  such  is  much  greater  whenever  " 
gill  breathing  becomes  difficult  or  impossible.  This  seems  to  be 
the  case  during  the  hot  season,  when  the  water  becomes  foul  from 
the  presence  of  decomposing  animal  or  vegetable  matter.  Semon 
recofcls  a  striking  illustration  of  this  in  the  case  of  a  partially 
dried-up  water  hole,  in  which  the  water  had  become  so  foul  that 
it  was  full  of  dead  fishes  of  various  kinds.  Fatal  as  these  condi- 
tions were  for  ordinary  fishes,  Neoceratodus  not  only  survived  but 
seemed  to  be  quite  healthy  and  fresh.  Such  observatiojis.  are  of 
exceptional  interest.  Not  only  do  they  afford  a  clue  to  the  condi- 
tions of  Hfe  which?  in  {nlTcourseof  time,  probably  led  to  lung-  ^^ 
breathing Jn>  Neoceratodus,  but  they  also  suggest  the  possibility" 
that  a  similar  environment  has%been  conducive^*)  the  evolution 
of  air-breathing  vertebrates*  from  gill-breathing  and  fish-like 
progenitors.  In^spite  of  its  pulmonary  respiration,  Neoceratodus 
more  closely  resembles  the  typical  Fishes  in  its  habits  than  any- 
other  Dipneustir^  It  livB|  all  the  year  round  in  the  water,  There 
is  no  evidence  that  it  ever  becomes  dried  up  in  the  mud,  oTpasses 
Into  a  summer  sleep  in  a  cocoon,  and  the  well-developed  condi- 


•I 

158  VERTEBRATE  ZOOLOGY 

tion  of  its  gills  suggest  that  these  organs  play  a  more  i  aportant 
role  in  breathing  than  in  either  Protopterus  or  Lepidos-yen" 

The  genus  Protopterus  (Fig.  95,  B)  has  a  wide  range  ovev  the  con- 
tinent of  Africa  and  consists  of  three  species,  P.  arsiectans,  P. 
/(Bthiopicus,  and  P.  dolloi.  These  fishes  inhabit  the  narshes  near 
rivers,  living  upon  frogs,  worms,  insects,  etc.,  ?fia,t  "abound  in 
marshy  jlaces  The  long_jsjender  tins  are  usecj^propably  as  tactile 
organs  though  they  may  help  in  locomotion  along  the  bottom. 
During  the  wet  season  they  live  and  breptne  much  as  does  Neocemto- 
dus. 

"In  the  dry  seasons,"  says  Bridge,  "the  marshes  in  which 
Protopterus  lives  become  dried  up,  and  to  meet  this  adverse 
change~iirfte~suTro\indings,  the  JhsTjJifernate^or  passes  into  a 
summer  sleep,  -until  the  next  rainy  season  brings  aBouTlconditions 
more  f a v or able^to  active  life.  Preparatory  to  this  summer  sleep, 
and  before  the  ground  becomes  too  .hard,  the  Fish  makes  its  way 
into  the  mud  to  a  depth  of  about  eighteen  inches,  and  there 
coils  itself  up  into  a  flask-like  enlargement  (Fig.  95,  D)  at  the 
bottom  of  the  burrow,  which  is  lined  by  a  capsule  of  hardened, 
mucus  secreted  by  the  glands  of  the  skin.  The  mouth  of  the 
flask  is  closed  by  a  capsular  wall  or  lid,  which  is  perforated  by  a 
small  aperture^  The  margins  of  this  aperture  are  pushed  inwards, 
so  as  to  form  a  tubular  funnel  for  insertion  between  the  lips, of 
the  Fish.  While  jejirapsnlecHn  its  cocoon  the  Fish  is  surrounded 
by  a  soft-slimy  mugus,  no  doubt  for  the  purpose  of  keeping  the 
skin  moist,  and  its  lungs  are^EEe  sole  .breathing  organs^  the  air 
pouring  from' the  open  mouth  of  the  burrow  through  the  hole 
in  the  lid  directly  to  the  mouth  of  the  animal.  The  nutrition 
of  the  dormant  Fish  is  effected  by  the  absorption  of  the  fat 
stored  about  the  kidneys  and  gonads,  somewhat  after  the  fashion 
not  unknown  in  the  fat-bodies  of  Insects  and  the  hibernating 
glands  of  Rodents." 

/  ]Lepidosiren  (Fig.  95,  C)  is  just  a  step  more  terrestrial  in  its  habits 
than  Protopterus  and  several  degrees  more  degenerate  than  the  latter. 
It  lives  in  swamps,  breathes  air  more  largely,  taking  several  breaths 
at  a  time  when  it  comes  to  the  surface.  In  the  dry  season  it  digs  a 
burrow  deeper  than  that  of  Protopterus.  It  even  lays  Its  eggs  in  deep 
burrows  in  the  ~t>la<&ppeaty--soil™0f  "the  swamps  in  which  it  lives, 


PISCES  159 

and  the  male  remains  in  the  burrow  guarding  the  eggs  till  they  hatch 
out  into  tadpole-like  larvae.  While  guarding  the  eggs  the  pelvic 
fins  of  the  male  act  as  accessory  external  gills,  the  fins  being  cov- 
ered with  numerous  vascular  filaments,  *^** 


GENERALIZED   AND   SPECIALIZED   TYPES   OF  FISHES   AND   THE 
AXIAL  GRADIENT  THEORY  OF  STRUCTURAL  RELATIONS 

In  nearly  all  of  the  orders  or  sub-orders  of  fishes  there  are  species 
that  have  retained  a  well-balanced  relation  of  head,  body,  and  tail; 
that  have  the  moderately  elongated,  cylindrical,  double-pointed 
shape;  with  smooth  body,  lacking  heavy  armature;  without  exaggera- 
tions of  the  fin-system ;  and  with  rather  dull  coloration.  The  dog-fishes 
among  elasmobranchs,  Polypterus  among  the  crossopterygians,  her- 
ring, pike,  trout,  killifish,  cod,  perch,  mackerel,  etc.,  among  the 
teleosts;  all  these  have,  to  a  more  or  less  complete  extent,  retained  the 
generalized  characters  of  the  ancestral  prototype  of  all  the  fishes. 
They  all  agree  quite  closely  with  the  very  ancient  types  that  have 
come  down  to  us  in  fossil  form.  All  of  these  generalized  types  are 
active,  predaceous  fishes,  with  an  abundant  supply  of  energy;  they 
are  youthful  in  the  physiological  sense. 

Specialization  in  fishes,  as  in  other  groups  of  vertebrates,  follows 
certain  definite  lines  and  results  in  several  types  of  structural  modifi- 
cation. One  of  the  commonest  of  these  is  the  eel-like  type,  which 
appears  in  all  of  the  dominant  sub-orders.  In  fishes  of  this  type  the 
body  is  greatly  elongated,  and  the  trunk  and  tail  are  apt  to  be  propor- 
tionately more  highly  developed  than  the  head.  The  head  remains 
small  (microcephalic)  and  exhibits  a  number  of  degenerate  features, 
such  as  small  eyes  and  imperfect  branchial  openings.  The  paired 
fins  are  usually  absent  and  the  median  fins  are  of  the  primitive,  un- 
differentiated  diphy cereal  type.  Perhaps  the  most  conspicuous 
example  of  reduced  head  is  seen  in  the  Hawaiian  eel,  Callechelyx  luteus, 
a  species  in  which  the  diameter  of  the  body  is  about  twice  that  of  the 
head.  Modern  types  of  eel-like  fishes  are  usually  scaleless,  but  some 
of  the  archaic  forms,  such  as  the  crossopterygian  species,  Calamichthys, 
are  heavily  scaled.  In  general,  the  eel-like  type  may  be  interpreted 
as  a  result  of  a  suppression  of  the  head  parts  and  a  consequent  relative 
increase  in  the  development  of  the  body  and  the  tail. 
'  The  antithesis  of  the  eel-like  type  is  that  in  which  the  head  parts 
are  abnormally  large  (megacephalic)  and  the  body  and  tail  relatively 


160  VERTEBRATE  ZOOLOGY 

suppressed.  Some  of  the  most  extreme  examples  of  this  condition 
have  just  been  called  to  our  notice  in  the  description  of  the  last  two 
sub-orders  dealt  with,  the  Pediculati  and  the  Plectognathi.  These 
types  of  structural  distortion  may  be  called  for  convenience  head-fish 
(megacephalic)  types.  Two  other  types  not  directly  related  to  the 
two  primary  types  just  mentioned,  but  more  or  less  closely  correlated 
with  them,  are:  first,  the  short,  high,  compressed  type;  and  the  second, 
the  low,  laterally  expanded  type.  Both  of  these  types  agree  in  having 
the  primary  axis  foreshortened  and  the  subordinate  axes  exaggerated. 
The  first  involves  an  exaggeration  of  the  dorso-ventral  (secondary) 
axis  at  the  expense  of  the  primary  and  tertiary  axes.  In  extreme 
cases  the  dorso-ventral  diameter  exceeds  the  length;  and  the  dorsal 
and  ventral  integumentary  elements,  such  as  fins  and  spines,  become 
greatly  lengthened  and  specialized,  as  in  Zanclus  (Fig.  86) .  The  sec- 
ond type  shows  a  dominance  of  the  tertiary  axis  (bilateral)  over  both 
primary  and  secondary  axes;  and  the  result  is  a  very  wide  type,  with 
expanded  and  specialized  pectoral  fins,  the  pelvic  fins  having  been 
relatively  suppressed  in  the  foreshortening  process  that  has  affected 
the  primary  axis. 

All  of  these  specialized  and  degraded  types  of  fishes  may  be  reduced 
to  two  categories: — a,  those  in  which  there  has  been  a  relative  sup- 
pression of  the  apical  parts  of  the  various  axes,  especially  the  primary 
axis,  accompanied  by  a  relative  emancipation  of  the  subordinate  axes 
from  the  dominance  or  control  of  the  primary  axis;  6,  those  in  which 
the  apical  parts  of  the  various  axes  have  become  relatively  highly 
specialized  or  exaggerated,  while  the  basal  elements  have  become 
relatively  suppressed. 

In  the  opinion  of  the  writer,  all  of  these  conditions  can  be  readily 
interpreted  as  the  morphological  consequences  of  growth-inhibiting 
agents,  acting  during  the  ontogeny  of  the  individuals.  The  morpho- 
logical equivalents  of  all  of  these  exaggerated  types  of  natural  fishes 
can  be  experimentally  simulated  by  the  use  of  inhibiting  agents 
applied  to  the  eggs  or  young  embryos  of  generalized  species  of  fishes. 
The  writer  and  other  investigators  have  performed  extensive  series  of 
experiments  with  the  eggs  of  Fundulus  (Fig.  79)  and  other  generalized 
types  of  teleosts,  using  a  wide  variety  of  growth-depressing  agents, 
such  as  anaesthetics,  low  temperatures,  heterogenic  hybridization, 
etc.  If,  for  example,  the  eggs  or  early  embryos  are  placed  for  limited 
periods  in  weak  sea-water  solutions  of  alcohol  or  potassium  cyanide, 


PISCES  161 

the  result  is  a  series  of  monsters,  ranging  from  those  in  which  only  the 
most  anterior  structures  (nostrils  and  eyes)  are  abnormal  to  blind  and 
nearly  headless  forms.  Between  these  extremes  are  types  in  which  the 
eyes  are  too  close  together  and  the  mouth  narrow  and  protruding; 
those  in  which  the  eyes  are  fused  into  a  single  median  cyclopian  eye 
and  the  mouth  is  a  sort  of  extended  proboscis;  and  those  in  which  the 
merest  rudiments  of  eyes  or  other  anterior  structures  are  developed. 
All  of  these  types  are  to  be  interpreted,  according  to  the  nomenclature 
of  Child,  as  the  results  of  differential  inhibition;  which  implies  that 
the  parts  of  the  body  that  normally  have  the  highest  rate  of  metab- 
olism and  are  the  first  to  differentiate,  are  the  parts  that  are  most 
readily  inhibited  by  growth-depressing  agents;  while  the  parts  that 
have  the  lower  or  lowest  metabolic  rates  are  least  affected  by  the 
same  agents. 

How  then  can  we  explain  the  type  in  which  the  posterior  parts  are 
relatively  inhibited,  and  the  anterior  parts,  together  with  the  apical 
parts  of  the  secondary  and  tertiary  axes,  are  relatively  more  highly 
differentiated?  These  conditions  become  instantly  intelligible  as  the 
result  of  another  type  of  experiment  with  Fundulus  eggs  and  em- 
bryos. If  eggs  are  placed  in  a  weak  solution  of  alcohol  or  cyanide  and 
are  allowed  to  remain  there  indefinitely,  the  rate  of  metabolism,  and 
consequently  of  development,  is  generally  retarded  for  a  time,  but 
gradually  a  process  of  acclimation  or  recovery  takes  place,  the  result 
of  which,  curiously  enough,  is  that  the  parts  that  primarily  were  most 
seriously  inhibited  are  the  first  to  become  acclimated,  and  recover 
more  completely  than  the  other  parts.  If  the  solution  be  made  strong 
enough  to  be  lethal  for  most  of  the  embryos,  a  few  of  the  hardiest  of 
them  undergo  a  very  limited  recovery,  involving  only  the  most  apical 
structures,  such  as  eyes.  Several  investigators  have  obtained,  in 
ways  similar  to  that  described,  embryos  that  consisted  of  nothing  but 
isolated  eyes;  and  it  is  very  common  to  find,  as  the  result  of  less  ex- 
treme measures,  embryos  that  consist  merely  of  heads  with  large 
rolling  eyes  and  provided  with  a  tiny  undifferentiated  appendage  that 
stands  for  the  rest  of  the  body.  Other  embryos  that  are  mainly  heads 
develop  in  addition  large  wing-like  pectoral  fins;  still  others  become 
broad  and  flat,  like  a  Skate,  or  high  and  compressed  like  a  Head-Fish, 
In  fact  a  good  assortment  of  exper  mental  monster  fish  embryos  will 
furnish  parallels  to  most  of  the  stock  types  of  form  distortion  seen 
in  the  specialized  and  degenerate  groups  of  fishes.  Of  course  none  of 


162  VERTEBRATE  ZOOLOGY 

these  embryos  differentiate  definitive  structures  or  take  on  hard  in- 
tegumentary coverings,  for  they  live  at  best  for  a  few  weeks  and  do 
not  acquire  the  adult  characters. 

But  what  in  nature  corresponds  to  the  growth-retarding  agents 
used  in  the  laboratory?  The  inhibitors  that  are  responsible  for 
racial  retardation  or  racial  senescence,  which  are  the  same,  are  in- 
ternal, and  are  probably  associated  with  an  aging  of  the  heredity 
chromatin  or  specific  germinal  protoplasm.  The  metabolic  rate  of 
the  germinal  elements  is  believed  to  lose  momentum  from  generation 
to  generation  and  from  age  to  age,  unless  rejuvenated  or  secondarily 
speeded  up  in  some  way.  In  the  senescent  species  we  must  conclude 
that  the  progressive  slow-down  of  the  germinal  metabolic  rate  is  ir- 
reversible and  that  these  forms  must  become  extinct  when  they  have 
gone  to  the  limit  of  their  differentiational  excesses.  The  generalized 
forms  may  be  looked  upon  as  racially  perpetually  young  or  ready  for 
any  new  processes  of  differentiation.  Whether  the  inhibiting  agents 
are  external  or  internal  the  same  types  of  morphological  distortion 
of  the  generalized  condition  result. 

Some  of  the  specific  cases  of  abnormal  structure  among  present- 
day  fishes  may  be  more  directly  attributable  to  the  external  condi- 
tions under  which  they  live  and  develop.  What  more  reasonable 
explanation  of  the  blind  cave  fishes  is  there  than  that  their  embryos 
have  been  victims  of  growth-depressing  agencies,  such  as  cold,  low 
oxygen  content  of  the  water,  or  darkness?  Similarly  many  of  the 
abysmal  fishes  could  be  explained  as  the  result  of  the  unfavorable 
developmental  conditions  of  the  sea  depths  (Fig.  74).  These  creatures 
are  for  the  most  part  either  eel-like  forms  or  Head-Fish  forms,  the  first 
of  which  may  be  attributed  to  differential  inhibition,  and  the  second, 
to  general  inhibition  followed  by  differential  recovery  of  apical  struc- 
tures. 

Similarly,  racial  senescence,  as  expressed  in  psedogenetic  forms,  may 
mean  that  certain  species  have  been  so  slowed  down  in  their  develop- 
mental momentum  that  they  are  unable  to  push  their  developmental 
processes  past  a  larval  period,  but  that  the  germinal  elements,  that 
belong  to  the  part  of  the  axis  with  the  lowest  rate  of  metabolism,  go 
on  and  become  mature,  thus  enabling  these  creatures  to  reproduce 
while  the  somatic  structures  are  still  larval  or  juvenile  in  character. 

High,  compressed  forms  appear  to  be  the  result  of  differential  in- 
hibition of  the  primary  axis  followed  by  differential  recovery  of  the 


PISCES  163 

anterior  end  of  the  primary  axis  and  of  the  secondary  or  dorso-ven- 
tral  axis,  especially  of  the  dorsal  (apical)  part  of  this  axis.  The  broad, 
flat  types  are  to  be  thought  of  as  distinctly  more  senescent  types  in 
which  the  whole  system  is  more  profoundly  inhibited,  but  in  which 
the  head  parts  and  those  nearest  to  the  head  recover  most  com- 
pletely, while  the  tertiary  axis,  especially  the  apical  parts  of  it,  ex- 
presses itself  at  the  expense  of  both  primary  and  secondary  axes. 

The  physiological  explanation  of  the  development  of  inert  structures 
like  armor,  heavy  spines,  and  even  of  massive  bones  and  flesh,  is  that 
a  lowered  rate  of  chemical  action  allows  of  the  precipitation  of  inert 
compounds,  which,  when  the  rate  is  higher,  would  be  used  up  in  dy- 
namic activity,  such  as  rapid  locomotion  or  rapid  growth.  Giant 
size  then  may,  on  this  theory,  be  just  as  definitely  a  product  of  racial 
sensecence  as  is  heavy  armor. 

In  this  volume  it  would  scarcely  be  appropriate  to  pursue  this 
theory  of  vertebrate  morphogenesis  further.  It  is,  however,  the 
writer's  opinion  that  the  theory  is  as  applicable  to  all  of  the  verte- 
brate classes  as  it  is  to  the  fishes.  The  parallel  between  the  condi- 
tions seen  in  the  teleost  fishes  and  in  the  birds  is  especially  close  and 
would  repay  detailed  examination. 

EGGS,  REPRODUCTION,  AND  BREEDING  HABITS  OF  FISHES 

The  eggs  of  living  species  of  fishes  vary  within  very  wide  limits  both 
in  size  and  in  form,  as  is  well  shown  in  Fig.  96.  The  largest  eggs  are 
those  of  some  Elasmobranchii,  which  compare  favorably  in  size  with 
those  of  birds.  They  contain  a  large  accumulation  of  yolk  and  have 
a  hard  chitinous  shell.  The  smallest  eggs  are  the  pelagic  eggs  of  many 
of  the  teleosts,  which  are  less  than  1  mm.  in  diameter.  Greatest 
egg-size  is  found  in  that  group  which  we  have  been  considering  the 
most  primitive;  least  egg-size,  in  the  most  highly  specialized  group. 
Are  we  justified  then  in  believing  that  the  primitive  fish  egg  was  of 
large  size  and  that  the  course  of  evolution  has  been  steadily  in  the 
direction  of  a  smaller  and  smaller  size  of  ovum?  Certain  other  con- 
siderations demand  a  negative  answer. 

Curiously  enough  both  the  largest  and  the  smallest  fish  eggs  are 
decidedly  telolecithal  and  show  an  advanced  type  of  meroblastic 
cleavage,  while  other  groups  of  fishes,  such  as  the  Chondrostei,  Holo- 
stei,  and  the  Dipneusti  have  eggs  of  medium  size  with  considerable 
yolk,  and  exhibit  various  degrees  of  incomplete  holoblastic  cleavage. 


164 


VERTEBRATE  ZOOLOGY 


FIG.  96. — Eggs  and  egg-cases  of  fishes.  A,  Bdellostoma,  egg-case;  B,  upper 
pole  of  same,  showing  hooks  and  micropyle  (after  Avers);  C,  Myxine  (after 
Steenstrup);  D,  a  process  of  same;  E,  Peiromyzon  marinus;  F,  Scyllium  (after 
Giinther).  G,  Raja;  H,  Heterodontus  (after  Giinther) ;  I,  Callorhynchus  (after 
Giinther) ;  J,  Ceratodus  (after  Lemon) ;  K,  Lepidosteus;  L,  Acipenser;  M,  Arius, 
showing  larva  (after  Gtinther) ;  N,  Serranus;  O,  Alosa;  P,  Blennius,  egg-capsules 
attached;  Q,  the  same  enlarged  (after  Guitel).  (From  Lankester,  after  Dean.) 


PISCES  165 

The  eggs  of  Amia  (a  holostean  ganoid)  show  perhaps  the  nearest  ap- 
proach to  complete  holoblastic  cleavage.  In  that  species  the  clea- 
vage furrows  run  meridionally  practically  from  pole  to  pole,  but  the 
equatorial  cleavage  furrows  in  the  vegetal  region  are  very  slow  in  ap- 
pearing, giving  to  the  early  cleavage  stages  quite  a  meroblastic  ap- 
pearance. The  egg  of  Lepidosteus,  another  holostean  ganoid,  shows 
an  incomplete  cleavage  of  the  vegetal  pole,  furrows  running  only  about 
to  the  equator  or  a  little  beyond.  Subsequently,  however,  the  fur- 
rows deepen  and  the  entire  egg  is  broken  up  by  cleavage  into  true  cells. 
Transitional  cases  of  this  sort  indicate  that  the  primitive  pro-fishes 
had  small  isolecithal  eggs,  like  that  of  Amphioxus,  and  that,  when  the 
first  fishes  invaded  the  region  of  the  rapid  currents,  they  had  to  lay 
their  eggs  on  the  bottom,  probably  attached  to  vegetation,  or  in  a 
prepared  nest  of  some  sort.  The  primitive  eggs  of  the  ganoids  and 
Dipnoi  are  covered  with  a  coating  of  sticky  jelly  and  are  laid  on  the 
bottom.  Accompanying  this  habit  there  was  an  increase  in  yolk 
accumulation  and  a  tendency  toward  the  telolecithal  condition  and 
meroblastic  cleavage.  That  the  modern  teleosts  are  all  meroblastic, 
even  when  the  eggs  are  very  small  and  have  only  a  minimum  amount 
of  yolk,  is  doubtless  due  to  their  derivation  from  ancestors  that 
had  a  very  large  yolk,  larger  probably  than  that  of  the  amphi- 
bian eggs  of  to  day.  Although  pelagic  fish  eggs  are  small  enough 
readily  to  admit  holoblastic  cleavage,  they  retain  the  extreme 
type  of  meroblastic  cleavage  characteristic  of  their  large-yolked 
ancestors. 

The  eggs  of  elasmobranchs,  the  largest  of  fish  eggs,  are  laid  singly 
or  in  pairs  at  varying  intervals  over  a  long  breeding  season.  In  some 
teleosts  the  number  of  tiny  eggs  laid  by  a  single  female  in  the  course 
of  a  short  breeding  season  of  a  few  days  reaches  into  the  millions,  the 
extreme  case  on  record  being  that  of  a  fifty-four  pound  Ling  that  laid 
over  twenty-eight  million  eggs.  Between  these  two  extremes  there 
are  all  intergrades.  When  the  eggs  are  large  and  few  are  laid,  chance 
methods  of  fertilization  cannot  be  relied  upon.  It  is  therefore  sig- 
nificant that  in  the  elasmobranchs  a  sort  of  copulation  occurs  during 
which  the  male,  by  the  use  of  claspers,  introduces  milt  into  the  ovi- 
ducts of  the  female  and  thus  accomplishes  internal  insemination  of 
the  eggs.  In  some  cases  gestation  is  also  internal,  but  this  is  doubt- 
less a  secondary  adaptation.  When,  on  the  other  hand,  the  eggs  are 
small  and  extremely  numerous,  fertilization  is  external  and  haphazard. 


166 


VERTEBRATE  ZOOLOGY 


The  males  and  females  simply  swim  about  in  schools,  emitting  eggs 
and  sperm.  The  eggs  are  fertilized  in  large  numbers  and  float  about 
near  the  surface  of  the  sea.  Only  a  small  percentage  of  them  complete 
their  development  to  hatching  and  large  numbers  are  eaten  as  larvae 
by  enemies.  Between  these  extremes  again  there  are  numerous  habit 
intergrades.  Some  teleosts  such  as  Gambusia  and  Anableps,  with 
comparatively  few  large  eggs,  practice  pairing  and  intromission  of 
sperm,  with  resultant  viviparity.  Members  of  the  same  family  such 


FIG.  97. — Development  of  Neoceratodus  forsteri.  A,  lens-shaped  blastula; 
B,  stage  with  semicircular  blastopore  (bl.  p,) ;  C,  later  stage  in  which  the  blastopore 
(bl.  p.)  has  taken  the  form  of  a  ring-like  groove  enclosing  the  yolk-plug  (ylk.  pi.)', 
D,  stage  in  which  the  narrow  medullary  groove  (blp.  sut.)  has  appeared  with 
the  rudiment  of  the  medullary  folds  (wed.);  E,  stage  in  which  the  medullary 
folds  (med.)  have  become  well  developed;  F,  later  stage  with  well-formed  head 
and  two  visceral  arches  (vise.)  and  rudiments  of  eye  (eye)  and  ear  (and.);  pron, 
mesonephros.  (From  Parker  and  Haswell,  after  Semon.) 

as  Fundulus  and  Cyprinodon,  that  have  smaller  and  more  numerous 
eggs,  practice  pairing,  the  males  clasping  the  females  with  their  dorsal 
and  anal  fins  during  the  process  of  egg  and  sperm  emission.  Thus  there 
is  less  chance  of  a  failure  of  insemination.  The  female  of  some  fishes 
such  as  the  black  bass,  sticklebacks,  etc.,  lay  eggs  in  a  nest  and  the  male 
follows  her  into  the  nest  and  inseminates  the  eggs.  The  male  pickerel 
follows  the  female  closely  and  fertilizes  the  eggs  as  soon  as  laid.  Many 
grades  and  modifications  of  this  habit  occur  that  gradually  lead  up 


PISCES 


167 


B 


C 


to  the  promiscuous  fertilization  of  eggs  when  numbers  of  males  and 
females  spawn  in  schools. 

It  seems  highly  probable 
that  the  habit  of  nesting, 
such  as  is  seen  in  A mia  and 
the  Dipneusti  is  close  to 
the  primitive  condition  and 
that  there  has  been  a  spe- 
cialization, in  one  direction, 
of  few  large  eggs  and  in- 
ternal fertilization,  and  in 
the  other  direction,  as  in 
fishes  living  in  open  waters, 
of  an  increase  in  numbers 
and  decrease  in  size  of  eggs 
and  consequent  haphazard 
fertilization.  For  primitive 
breeding  habits  therefore,  I 
would  be  inclined  to  look 
to  Amia  and  the  Dipneusti, 
where  the  conditions  are 
not  so  very  different  from 
those  in  Amphibia. 

The  fundamental  embry- 
ological  changes  following 
holoblastic  and  those  fol- 
lowing meroblastic  cleav- 
age are  decidedly  different, 
and  an  example  of  each,  FIG.  98. — Development  of  a  teleost  (Sal- 
chosen  from  the  fishes,  may  mon.)  A,  four-cell  stage;  B,  multicellular 
,  .„  ,  ,  blastoderm  (an  early  blastula  stage) ;  C,  blas- 

serve     to  S0me      toderm  (6L)  beginning  to  overgrow  the  yolk; 

D,  gastrulation  beginning  and  germ-ring  (r) 
formed;  E,  and  F,  embryo  formed  by  concres- 
cence of  germ  ring  and  germ  ring  one-third 
and  two-thirds  around  yolk;  G  and  H,  early 
and  advanced  embryos  (emb.}  with  blastoderm 
surrounding  yolk-sac  (y.  s.);  I,  just  hatched 
larva  with  remains  of  yolk-sac  (y.  s.).  (From 
Parker  and  Haswell,  after  Henneguy.) 


H 


developmental 


important 
principles. 

The  case  of  Neocerato- 
dus,  the  most  primitive  of 
the  modern  Dipneusti  may 
be  taken  to  illustrate  holo- 
blastic cleavage  and  its  ap- 
propriate type  of  gastrulation. 


As  described  by  Semon  for  the  rela- 


168  VERTEBRATE  ZOOLOGY 

lively  small  egg  of  this  fish,  the  first  two  cleavages  are  meridional  and 
divide  the  egg  completely  into  four  equal  blastomeres.  The  third 
cleavage  is  also  meridional;  but  the  fourth  is  equatorial,  cutting  off 
eight  micromeres  and  eight  macromeres.  The  micromeres  multiply 
more  rapidly  than  the  macromeres  and  produce  a  blastula  with  num- 
erous small  cells  at  the  animal  pole  and  fewer  large  cells  at  the  vegetal 
pole  (Fig.  97).  The  cavity  contains  many  rather  loose  yolk  cells. 
Gastrulation  occurs  by  invaginating  the  endoderm  to  one  side  of  the 
yolk  mass.  Embryo  formation  is  very  like  that  of  the  Amphibia. 

The  extreme  type  of  meroblastic  cleavage  is  seen  in  the  teleosts 
(Fig.  98).  Here  the  protoplasmic  parts  of  the  egg  during  the  processes 
of  maturation  migrate  to  the  animal  pole  and  round  up  into  a  yolk- 
free  hemisphere.  This  hemisphere  undergoes  cleavage,  forming  a 
lens-shaped  cap  of  cells.  This  so-called  embryonic  disk  then  pro- 
ceeds to  surround  the  yolk  by  a  process  of  overgrowth  or  peripheral 
spreading  of  the  germinal  disk,  meanwhile  differentiating  an  embry- 
onic head.  The  germ-ring  proceeds  past  the  equator  of  the  egg  and 
then  grows  together  as  it  becomes  narrower  to  form  the  embryonic 
axis.  Finally  the  ring  closes  completely,  its  substance  having  con- 
cresced  to  form  the  embryonic  body!  This  process  of  embryonic 
concrescence  is  characteristic  of  all  vertebrate  embryos  in  which 
meroblastic  cleavage  occurs.  There  are  all  connecting  stages  be- 
tween the  type  of  development  seen  in  Neoceratodus  and  that  seen 
in  the  typical  teleost. 

APPENDIX  TO  FISHES 

THE    OSTRACODERMI 

Incidental  mention  has  already  been  made  to  the  Ostracodermi 
as  an  early  specialized  group  of  pro-fishes.  They  are  Palaeozoic 
forms  which  have  a  wide  range  of  characters,  so  wide  in  fact  that  it  is 
doubtful  whether  it  is  justifiable  to  place  them  in  a  single  group.  If 
we  give  the  group  as  a  whole  the  value  of  a  class  coordinate  with 
Cyclostomata  and  Pisces,  we  may  be  justified  in  dividing  this  " class" 
into  a  number  of  orders. 

ORDER  I.  HETEROSTRACI 

Of  these  forms  we  have  little  knowledge  except  of  their  external 
features.  They  were  evidently  broad,  flat  creatures,  more  like  our 


PISCES 


169 


present  Skates  and  Rays  than  anything  else  and  probably  having 
similar  but  more  sluggish  habits.    Thelodus  and  Lanarkia,  the  restored 


FIG.  99. — A  group  of  ostracoderms  and  their  kin.  A  and  G  directly  after 
Patten;  the  rest  from  Patten,  after  Traquair.  A,  Bothriolepis  canadensis;  B, 
Thelodus;  C,  Lanarkia;  D,  Birkenia;  E,  Lasanius;  F,  Drepanaspis;  G,  Cephal- 
aspis.  (Redrawn  after  Patten.) 

outlines  of  which  are  shown  in  Fig.  99  B  and  C,  seem  to  have  had  an 
exoskeleton  composed  of  "  a  uniform  covering  of  hollow  pointed  spines, 
devoid  of  basal  plate,  and  open  below."  These  spines  are  composed 


170  VERTEBRATE  ZOOLOGY 

of  dentine  covered  with  ganoin.  The  forward  part  of  the  body 
has  lateral  fin-folds  of  a  very  primitive  type  and  the  tail  is  pro- 
vided with  a  very  primitive  heterocercal  tail.  Elasmobranch  char- 
acters are  quite  obvious  here  and  it  is  believed  that  this  group 
represents  a  specialized  bottom-feeding  adaptive  radiation  from 
the  most  primitive  shark  types  of  the  lower  Ordovician  or  Cam- 
brian times. 

Another  genus  that  has  been  placed  in  this  order  is  Drepanaspis 
(Fig.  99,  F).  This  creature  is  much  more  highly  specialized  in  its 
exoskeleton  than  is  Lanarkia  and  furnishes  a  transition  between  the 
latter  and  the  more  heavily  armed  condition  of  the  Pteraspidae,  a 
group  that  reaches  the  climax  of  armature  in  this  order.  Drepanaspis 
is  also  a  broad  flat  form  with  lateral  fin-folds  and  a  heterocercal  tail. 
The  whole  body  is  covered  with  a  continuous  armature  of  tile-like 
plates,  some  of  these  plates,  especially  a  median  dorsal,  a  median 
ventral,  and  paired  laterals,  being  especially  conspicuous.  The  rest 
of  the  body  is  covered  with  smaller  tessellated  plates  of  various 
sizes. 

In  the  genus  Pteraspis  the  armor  over  the  cephalothorax  is  much 
simplified  by  dropping  out  all  but  the  largest  plates,  and  by  the  de- 
velopment of  a  rostral  plate.  The  tail  is  decidedly  fish-like  and  covered 
with  rhomboidal  plates  much  like  those  of  the  lobe-finned  ganoids  or 
the  modern  gar-pike.  A  strong  dorsal  spine  is  a  conspicuous  feature 
of  this  species. 

ORDER  II.  OSTEOSTRACHI 

This  order  resembles  the  Heterostraci  in  having  the  anterior  body 
covered  with  a  solid  armor  and  the  tail  free  to  move.  They  differ 
from  the  Heterostraci  in  having  bony  plates  instead  of  mere  calcifica- 
tions, in  having  a  dorsal  fin,  and  in  having  eyes  median  instead  of 
lateral  in  position.  The  carapace  reminds  one  strongly  of  that  of  the 
King  Crab  (Limulus)  and  its  extinct  relatives,  but  the  resemblance  is 
probably  merely  a  superficial  one  due  to  similarity  of  habits.  They 
apparently  had  "a  grovelling  bottom-feeding,  sluggish  habit  of  life," 
in  contrast  with  the  active  predaceous  life  of  their  free-living,  shark- 
like  ancestors.  They  played  out  their  string  of  specialization  and 
became  extinct  during  Devonian  times. 


PISCES 


171 


ORDER  III.  ANASPIDA 

This  order  is  established  to  contain  two  genera  of  fish-like  forms, 
Birkenea  (Fig.  99,  D)  and  Lasanius  (Fig.  99,  E)  about  which  there  is 
only  fragmentary  information,  and  which  are  placed  among  the  Ostra- 
codermi  only  provisionally  till  more  knowledge  of  their  characters  is 
forthcoming. 

ANTIARCHI 

This  group,  placed  originally  among  the  Ostracodermi,  is  now  given 
class  value  and  separated  from  the  latter.  It  is  from  such  forms  as 
Bothriolepis  (Fig.  99,  A)  that  Patten  would  derive  the  vertebrates 
through  the  connecting  link  of  the  extinct  sea-scorpions.  Like  the 
Ostracodermi,  the  Antiarchi  have  a  heavily  armored  carapace  and  a 
free  fish-like  tail.  The  carapace  is,  however,  differentiated  into  a 
head-shield  and  a  thoracic  shield.  According  to  Patten  this  creature, 
which  is  fish-like  in  most  of  its  characters,  has  the  lateral  paired  jaws 
of  the  Arthropoda  and  has  a  brain  more  arthropodan  than  chor- 
date.  Perhaps  the  most  characteristic  feature  of  the  group  is  the  pair 
of  appendages  that  is  jointed  to  the  cephalic  carapace  and  reminds 
one  at  the  same  time  of  arthropodan  appendages  and  of  vertebrate 


FIG.  100. — Coccosteus  decipiens.  Side  view,  restored.  A,  articulation  of  head 
with  trunk;  DB,  cartilaginous  basals  of  dorsal  fin;  DR,  cartilaginous  radials  of 
dorsal  fin;  H,  haemal  arch  and  spine;  MC,  Mucous  canals;  N,  neural  arch  and 
spine;  V,  median  unpaired  plate  (?)  of  hinder  ventral  region;  VB,  basals  of  pelvic 
fin;  VR,  radials  of  pelvic  fin.  (From  Dean,  after  Smith- Woodward.) 

limbs.  A  lateral-line  system  of  chordate  type  is  quite  clearly  shown. 
It  has  been  suggested  that  the  paired  appendages  may  be  the  fore- 
runners of  the  vertebrate  jaws.  This,  however,  seems  rather  far- 
fetched. 

As  has  been  previously  stated  this  group  is  not  only  a  highly  special- 
ized one,  but,  in  its  heavily  armed  body  and  evidently  sluggish  habits, 


172  VERTEBRATE  ZOOLOGY 

it  bears  evidences  of  senescence.  Hence  it  is  not  at  all  the  kind  of 
group  which  we  would  expect  to  give  rise  to  the  generalized  verte- 
brates of  the  early  period.  It  is  rather  to  be  thought  of  as  a  preco- 
ciously specialized,  bottom-feeding  derivative  of  some  early  fish-like 
group. 

ARTHRODIRA 

The  Arthrodira  are  real  armored  Fishes,  whose  exact  relations  are 
unknown.  It  is  not  unlikely  that  they  represent  an  adaptive  radia- 
tion from  the  primitive  lobe-fin  ganoids.  They  are  provided  with  a 
heavy  cephalic  and  a  separate  thoracic  shield  and  have  a  typical  fish 
body  of  primitive  structure.  The  genus  Coccocteus  (Fig.  100)  is  a  good 
example  of  the  group. 


CHAPTER  VI 
CLASS  III.    AMPHIBIA 
PRESENT  AND  PAST  STATUS 

The  question  of  the  origin  of  the  Amphibia  involves  the  whole  prob- 
lem of  the  beginnings  of  land  life  among  the  vertebrates  and  the  radi- 
cal evolutionary  changes  that  have  occurred  as  adaptations  for  an  en- 
tirely new  mode  of  lifa  While  the  aquatic  habitat  may  be  said  to  be 
a  comparatively  uniform  and  constant  one,  only  slightly  influenced 
by  seasonal  changes,  the  terrestrial  life,  especially  in  temperate  re- 
gions, involves  a  wide  range  of  changing  conditions. 

It  has  already  been  noted  that  the  fishes  had  shown  marked  tend- 
encies to  adopt  various  methods  of  invading  the  air-breathing  realm, 
some  for  the  sake  of  avoiding  the  respiration  of.  toot  much  CO2  and 
other  poisonous  gases  in  stagnant  waters,  _some  to  tide  over  periods 
of  drought,  and  still  others  for  the  purpose  of  enabling  them  to  climb 
out  of  the  water  for  food  (the  climbing-perch,  etc.).  It  must  have 
been  in  association  with  conditions  resembling  these  that  the  first  true 
land  vertebrates  were  evolved,  v  - 

The  Amphibia  are  undoubtedly  the  most  primitive  land  verte- 
brates, but  it  is  coming  to  be  believed  that  the  first  Reptilia  trod 
closely  upon  their  heels.  The  Reptilia  were  much  more  truly  land 
vertebrates  than  were  the  Amphibia,  for  the  Amphibia  are  tied  down 
to  the  aquatic  medium  during  at  least  the  developmental  period,  in 
most  groups,  and  during  the  entire  life  cycle,  in  others.  Fundamen-  • 
tally  the  Amphibia'  are  aquatic  because  their  developmental  processes/ 
are  aquatic.  Only  a  few  of  the  most  highly  specialized  modern  Am- 
phibia lay  their  eggs  out  of  water,  -and  these  have  adopted  various 
unique  brooding  habits,  which  are  at  best  mere  developmental  make- 
shifts as  compared  with  the  methods  employed  by  the  reptiles  with 
their  amnion  and  allantois. 

The  Arnphibia  have  never  attained  the  heights  of  success  and  of 
dominance  in  nature  that  has  been  attained  by  fishes,  reptiles,  birds, 
or  mammals.  Possibly  this  lack  of  complete  success  has  been  the  re- 

173 


174  VERTEBRATE  ZOOLOGY 

suit  of  their  somewhat  anomalous  lives,  involving  the  necessity  of  an 
amphibious  environment.    They  are  forced  to  occupy  a  narrow  strip 
of  territory  between  the  waters  and  the  dry  land,  a  prey  to  the  domi- 
nant denizens  of  the  waters  (fishes)  on  the  one  hand  and  to  the  vari-  | 
ous  more  vigorous  enemies  on  the  land  (reptiles,  birds,  and  mammals)  ! 
on  the  other.     If  hard  pressed  in  one  environment  the  amphibian 
may  seek  the  other;  and  this  has  saved  him  from  complete  extinction. 

The  Amphibia  to-day  are  represented  largely  by  a  single  highly  spe- 
cialized order,  the  Anura^ (frogs  and  toads),  that  have  undergone 
within  comparatively  recent  times  a  wonderfully  elaborate  adaptive 
radiation  into  a  great  variety  of  habitat  complexes.  But  for  the 
Anura  the  modern  Amphibia  would  be  largely  unknown,  for  the  sala- 
manders, newts,  and  csecilians  are  furtive,  inconspicuous  forms  that 
have  sought  safety  irf  the  liidden  nooks  and  crannies  of  the  world 
environment  and  persist  through  their  extremely  retiring  habits. 

At  one  time,  however,  the  Amphibia  occupied  a  comparatively 
honorable  place  in  nature.  They  reached  in  some  cases  almost  giant 
size  and  evidently  were  active  and  predaceous  creatures.  Their  wane 
began  with  the  rapid  rise  of  the  Reptilia,  which,  as  a  group,  became 
much  more  completely  adjusted  to  land  life  than  did  the  Amphibia. 

THE  ORIGIN  or  THE  AMPHIBIA 

It  is  now  generally  admitted  that  the  Amphibia  arose  as  a  lateral 
branch  from  a  very  early  group  of  "lobe-fin"  ganoids  (Crossopterygii). 
The  time  of  emergence  of  the  first  Amphibia  appears  to  have  been 
about  Middle  Devonian,  the  period  when  the  fishes  gained  their 
earliest  pronounced  ascendancy  and  when  all  of  the  available  habi- 
tat complexes  were  occupied.  The  earliest  trace  of  amphibian  life 
is  a  single  footprint  (Fig.  101)  of  a  three-toed  species  (Thinopus  an- 
tiquus)  found  in  the  Upper  Devonian  shales  of  Pennsylvania.  *  This 
foot  though  primitive  was  a  true  foot;  not  a  fin.  It  is  therefore  prob- 
able that  there  were  many  transitional  stages  from  the  fin  to  the  foot 
which  are  beyond  our  ken,  and  that  the  transition  occupied  at  least 
thousands  of  years.  The  skeletal  structure  of  the  lobe-fin  ganoid 
paired  fins,  especially  that  of  the  pectorals,  is  quite  hand-like  in  ar- 
rangement ;  so  that  a  dropping  off  of  the  f ririge-like  fin  portion  would 
leave  a  structure  quite  like  a  hand  with  three  or  more  fingers  (Fig.  102, 
C,  D,  E).  The  dropping  of  the  fin-fringe  may  have  happened  quite 
suddenly  in  the  process  of  evolution  of  some  group — possibly  by  a 


AMPHIBIA  175 

single  mutation.  The  rest  of  the  change  would  be  one  of  gradual  func- 
tional adjustment.  It  was  noted  that  the  present-day  "lobe-fins" 
use  the  pectoral  fin  like  a  land  limb  in  that  they  support  the  weight 
upon  it  while  resting  on  the  bottom;  so  a  func- 
tional change  may  readily  have  preceded  tha 
radical  structural  change.  This  theory  of  the 
origin  of  the  amphibian  pentadactyl  limb  is- 
well  shown  in  Fig.  102,  A,  B. 

Palaeographers  inform  us  that  the  climatic 
conditions  of  the  Upper  Devonian  were  such 
as  to  encourage  the  development  of  land  life 
on  the  part  of  fishes  living  in  inland  waters. 
There  were  periods  of  warmth  and  heavy 
rainfall  followed  by  long  periods  of  drought, 
which  became  progressively  more  prolonged. 

Such  conditions  would  tend  to  drive  a  large  J£  ^  ~  ™ 
proportion  of  the  non-air-breathing  fishes  from  Thinopus  antiquus,  with 
the  fresh  waters  and  to  give  their  place  to  the  two  fu%  formed  digits, 
air-breathing  crossopterygians  and  their  kin.  jjj11  and'  ^  ^gjble  ^-udi 
With  increasingly  prolonged  dry  seasons-  the  merit  of  a  fourth,  IV. 
activating  habits  qjf  the  early  lung-breathing  Upper  Devonian  of  Penn- 

ri"1  .  -I    •       -i  Qvlvanm         lx£  naf.nral   aiva 

fishes  proved  inadequate  and  it  became  neces- 
sary for  the  animals  to  live  an  active  life  in  the 
air  and  to^get  their  food  on  the  land.  It  is  probable  that  although 
many  early  lung-breathing  fishes  made  the  beginnings  of  adaptation 
to  true  land  life,  only  one  type  fully  succeeded  and  became  the  first 
true  Amphibia,  the  ancestors  of  all  of  those  to-day  living. 

Adaptive'  Changes  Incident  to  Life  on  the  Land. — The  change 
from  aquatic  to  terrestrial  Jife  has  been  the  greatest  evolutionary 
crisis  in  vertebrate  history..  No  other  environmental  change  pos- 
sible for  animals  requires  so  radical  ,an  alteration  of  developmental 
and  nutritional .  (in  4he  broadest  sense)  mechanisms;  Changes 
from  salt  to  fcesh  water,  from  shallows  to  abysses,""  from  surface 
to  subterranean,  arboreal  or  aereal  life,  involve  much  less  funda- 
mental alterations  than  does  that  from  water  to_ land;  .which  is, 
strictly  speaking,  rather  a  change  from  wafygp-4o  air.  Naturally  the 
most  important  changes  had  to  do  with  respiration,  circulation,  and 
locomotion.  Changes ^of  secondary  value  concern  the  altered  specific 
gravity,  the  more  proitaunced  changes  in  temperature,  the  tendency 


176 


VERTEBRATE  ZOOLOGY 


toward  dessication,  the  differences  in  visibility  through  water  and  air, 
and  the  differences  in  conduction  of  sound  waves.  In  the  true  ter- 
restrial forms  (reptiles,  birds  and  mammals)  special  adaptations  for 
making  possible  embryonic  development  in  the  air  have  been  ac- 
quired, but  not  so  in  the  Amphibia,  which  develop  through  to  the 


FIG.  102. — To  illustrate  the  change  from  fin-type  of  appendage  to  the  foot- 
type,  and  the  reverse  or  secondary  adaptation  of  the  foot- type  into  the  fin- type. 

The  upper  figures  (A  and  B)  represent  the  theoretic  mode  of  metamorphosis  of 
the  lobe-fin  of  the  Crossopterygian  fish  (A)  into  the  foot  of  the  amphibian  (B) 
through  the  loss  of  the  dermal  fringe  border  and  rearrangement  of  the  cartilag- 
inous supports  of  the  lobe.  C,  D,  E,  show  the  skeletal  support  of  the  two  types  of 
fin;  C,  the  lobe-fin  with  the  fin-fringe;  D,  the  lobe-fin  without  the  fringe;  and  E, 
the  foot-stage  as  seen  in  an  early  Carboniferous  amphibian.  F,  G,  H,  show  the 
secondary  reversed  evolution  of  the  five-rayed  limb  (F)  of  a  land  reptile  into  the 
fin  or  paddle  of  an  ichthyosaur  (G,  H).  (Redrawn  after  Osborn.) 

adult  condition  in  the  water,  undergoing  a  rather  sudden  metamor- 
phosis from  the  aquatic  to  the  terrestrial  physiology.  These  various 
primary  and  secondary  changes  in  structure  will,  when  listed,  serve 
to  indicate  the  differences  between  the  Fishes  and  the  Amphibia!-1- 
1.  Respiration. — If  we  go  back  to  the  lobe-finned. ganoids  for  the 
ancestors  of  the  Amphibia,  we  find  a  double  respiratory  system, 
branchial  and  pulmonary,  with  the  pulmonary  playing  an  accessory 
or  secondary  role.  In  times  of  extreme  drought  or  extreme  foulness 
of  the  water  the  branchial  respiration  was  held  in  abeyance  and  the 
pulmonary  used  almost  exclusively.  The  branchial  respiration  func- 
tions entirely  in  early  life  and  it  is  only  w'th  assumption  of  adult 


AMPHIBIA  177 

form  that  the  air-bladder  comes  to  be  greatly  in  demand.^  In  the  on- 
togeny of  modern  Amphibia  we  find  this  sequence  repeated,  for  the 
young  amphibian  is  purely  aquatic,  and  air-breathing  comes  only 
when  the  larva  metamorphoses  into  the_youn^_adiilt.  - 

2.  Circulation. — The  fish  type  of  circulation  is  built  primarily 
along  lines  laid  down  by  branchial  respiration. "  /The  heart  is  purely 
venous  in  its  blood  content  and  pumps  blood  forward  and  through 
the  branchial  arches.    This  involves  as  many  pairs  of  branchial  arches 
as  there  are  paired  functional  afferent  vessels  carrying  blood  to  the 
gills,  and   efferent^  vessels  ^carrying    the  Derated   blood    from    the 
gills  to  the  dorsal  aorta*-/  In  the  evolution  from'the.fish  to  the  am- 
phibian type  the  principal  changes  in  the  circulatioirfrave  to  do  with 
the'  branchial  arches,,  which  cease  to  have  a  value  as  such,  and  their 
profound  remodelling  into  blood  vessels  that  fit  into  an  air-breathing 
physiology.    The  branchial  vessels  of  lobe-finned  ganoids  and  of  lar- 
val amphibians  consist  of  four  pairs,*"  the  first  pair  becomes  the-ca- 
rotid  arteries;  that  supply  the  head,  the  second  becomes  the  systemic 
arches  that  supply  most  of  the  body,  the  third  disappears,  and  the 
fourth  becomes  mainly  the  pulmonary  arches.    It  is  of  interest  to  note 
that  in  all  lung-breathing  fishes  the  lungs"  are  supplied  from  a  branclT 
of  the  fourth  branchial  arch.     In  most  Amphibia  a  branch  of  the 
fourth  arch  becomes  cutaneous,  for  the  skin  respiration  is  almost  as 
important  as  the  pulmonary.     The  heart  becomes  three  chambered, 
the  auricle  dividing  into  a  systemic  half  and  a  pulmonary  half.    The 
single  ventricle  receives  both  arterial  and  venous  blood,  but  there  is 
very  little  admixture  of  the  two. 

3.  Locomotion. — In  fishes  the  chief  locomotor  organs  are  thejbaii, 
and  the  median^fins.     The  paired  fins  are  used  largely  for  balancing; 
in  lobe-finned  ganoids  they  are  used  to  support  the  head  when  rest- 
ing on  the  bottomX  Naturally  then  we  should  expect  the  most  radi- 
cal change  to  concern  the  loss  of  tail  fins  and  other  median  fins,  on 
the  one  hand,  and  the  development  «f  feet,  on  the  other.  \  The  median 
and  caudal  fins  appear  in  the  amphibian^larvae"  and  persist  in  a  re- 
duced form  in  certain  persistently  aquatic  amphibia,  which  are  prob- 
ably no  more  than  permanent  larvae  (psedogenetic)  „    When  the- fins 
do  appear  they  are  mere  soft  folds  of  the  ^kin  without  any  true  skel- 
etal supports,  and  they  never  become  regionally  specialized,  but  re- 
main in  the  ancient  diphycercal  fyfa$.    In  the  land  salamanders  the 
tail-fin  is  lost  but  the  tail  persists.    In\the  Anura,  as  well  as  in  the 


178 


VERTEBRATE  ZOOLOGY 


caecilians,  the  tail,  formed  fully  in  the  larvae,  is  secondarily  resorbed 
during  metamorphosis. 

The  change  from  paired  fins  to  paired  limbs  is  not  so  radical  as 
it  was  once  supposed.    Thanks  to  the  discovery  of  the  limb-skeleton 


FIG.  103. — Pectoral  fin  of  extinct  crossopterygian,  Sauripterus  iaylori.  '  cl, 
clavicle;  co,  coracoid;  H,  humerus;  R,  radius;  Sc,  scapula;  Scl,  supra  clavicle; 
u,  ulna.  (From  Lull,  after  Gregory.) 

of  some  of  the  early  crossop- 
terygians,  e.  g.  the  pectoral 
fin  of  Sauripterus  taylori 
from  the  Upper  Devonian 
(Fig.  103),  it  is  not  difficult 
to  see  how  a  fin  could  be- 
come a  foot.  Note  that 
this  fin,  minus  the  fringe, 
is  a  hand-like  structure, 
with  humerus,  radius,  and 
ulna,  wrist,  and  several  fin- 
gers. The  shoulder  girdle 
of  Sauripterus  is  also  part 
for  part  homologous  with 
that  of  an  amphibian. 
Some  changes  in  relative 
sizes  of  the  elements  and  a 
reduction  in  the  number  of 
repeated  parts  would  give 

FIG.  104. — Development  of  the  Hud  foot  of  a  salamander,  Triton  tceniatus. 
1-5,  first  to  fifth  digits;  A-F,  seven  stagr*  from  the  simple  limb-bud  to  the  defin- 
itive foot.  (From  Lull,  after  Rabl.) 


AMPHIBIA  179 

an  amphibian  hand.  The  footprint  of  Thinopus,  the  earliest  am- 
phibian trace,  does  not  reveal  any  of  the  skeletal  parts,  but  it  is 
likely  that  onlytwo  fingers  were  fully  developed  and  two  others 
partially  separated.  In  ^Che  development  of  the  amphibian  foot 
as  shown  in  Fig.  104  the  three-fingered  condition  persists  till_rather 
late  in  development;  then  a  fourth  finger  appears  well  down  on 
the  ulnar  side  of  the  hand  and  a  rudiment  of  the  fifth  (the  little 
finger)  appears  as  a  mere  bump.  The  thumb,  index,  and  second 
finger  seem  to  be  phylogenetically  the  oldest  digrbs,  and  this  is  im- 
portant in  connection  with  the  loss  of  fingers  in  other  vertebrates;  for 
it  is  always  the  latest  to  develop  that  is  first  lost.  With  the  substi- 
tution of  a  foot  for  a  fin  the  kind  of  movement  becomes  profoundly 
altered.  Instead  of  a  mere  paddling  back  and  forth,  a  variety  of  move- 
ments are  necessary  and  thus  the  old  myotomic  musculature  becomes 
decidedly  modified  until  nearly  all  traces  of  the  segmental  arrange- 
ment of  muscles  are  lost. 

4.  Changes  Due  to  Increased  Specific  Gravity. — When  the  ani- 
mal comes  from  the  water  to  the  land  it  is  relatively  heavier;  hence 
there  is  need  of  a  more  rigid  skeleton,  stronger  limb  girdles  and  limb 
skeleton.    This  is  accomplished  by  more  complete  ossification  of  the 
parts  of  the  skeleton  that  bear  the  most  weight^   There  is  likely  also 
to  be  a  reduction  of  dead  weight,  such  as  exoskeletal  parts.    In  modern 
Amphibia  the  exoskeleton  has  disappeared  .complete1  y  (except  in  csecil- 
ians,  where  it  is  rudimentary),  but  in  the  stegocephalians  the  head 
armor  persisted,  while  that  of  the  rest  of  the  body  largely  disappeared. 

5.  Responses  to  Seasonal  Changes  of  Temperature  are  much  more 
necessary  on  land  than  on  water.     Nearly  all  Amphibia  hibernate 
during  winter  either  by  burying  themselves  in  the  earth  or  in  water. 

6.  Changes  for  Avoiding  Dessication. — If  a  fish  is  taken  out  of 
water  it  soon  dries  up  on  the  surface  and  becomes  stiff.    Amphibia, 
however,  have  abundant  skin  glands,  secreting  moist  mucus  which 
keeps  the  skin  in  proper  condition  to  perform  a  respiratory  function 
and  helps  it  to  retain  its  flexibility.     Some  of  the  Amphibia  have 
rudimentary  lungs  and  respire  almost  exclusively  through  the  skin. 

7.  The  eyes  change,  especially  in  the  shape  of  the  lens,  which 
becomes  flattened  instead  of  spherical. 

8.  An  external  sound  receptor  appears  in  the  form  of  a  tympanic 
membrane,  which  is  in  communication  with  the-nfiier  ear  through  a 
columella,  a  bo^y  apparatus  that  vibrates  with  the  ear  drum. 


180 


VERTEBRATE  ZOOLOGY 


THE  EXTINCT  AMPHIBIA 

All  of  the  early  Amphibia  of  the  coal-measures  (Upper  Carbonifer- 
ous) are  classed  as  Stegocephali,  and  it  is  customary  to  divide  the 
Class  Amphibia  into  two  sub-classes:  (1)  Stegocephali  (with  dermal 
armor,  especially  on  the  head);  (2)  Lissamphibia  (modern  and  ex- 
tinct Amphibia  with  smooth  bodies,  devoid  of  heavy  dermal  armor). 

SUB-CLASS  I.  STEGOCEPHALI 

The  earliest  actual  skeletal  remains  of  Amphibia  were  found  in  the 
Edinburgh  Coal-Measures,  which  belong  to  Lower  Carboniferous 


FIG.  105. — Group  of  Extinct  Amphibia,  A  and  B  from  the  Carboniferous; 
C  and  F,  Permo-Carboniferous.  A,.  Pytonius;  B,  Amphibamus;  C,  Cacops; 
D,  Cricotus;  E,  Diplocaulus;  F,  Eryops.  (Redrawn  after  Osborn,  following 
restorations  of  Gregory  and  Decker t.) 

times.  These  forms,  Loxomma  and  Pholidogaster,  are  not  transitional 
but  fully  modified  Amphibia.  There  is  every  reason  to  believe  there- 
fore that  the  earliest  Amphibia  arose  back  in  the  Devonian, 


AMPHIBIA  181 

The  Amphibia  apparently  did  not  find  the  climatic  conditions  of  the 
Lower  Carboniferous  especially  adapted  to  them  and  did  not  really 
become  a  successful  and  dominant  race  till  during  the  Upper  Carbon- 
iferous and  Permian  times,  when  their  great  deployment  and  first 
adaptive  radiation  ccurred. 

The  first  Amphibia  (Fig.  105,  A)  were  probably  small-headed,  long- 
bodied  forms  with  fish-like  appearance,  resembling,  doubtless,  our 
modern  newts  and  salamanders.  During  the  Upper  Carboniferous, 
however,  there  was  an  adaptive  radiation  resulting  in  the  develop- 
ment of  large-headed,  short-bodied  types,  more  or  less  resembling 
our  frogs  and  toads  (Fig.  105,  B),  but  without  jumping  legs.  There 
also  appeared  some  broad,  flat  types  (Fig.  105,  E)  with  reduced  limbs 
that  must  have  been  bottom-feeders  (e.  g.,  Diplocaulus). 

Three  orders  of  Stegocephali  are  distinguished: 

Order  I.  Stegocephali  Leptospondyli. — This  group  is  characterized 
by  pseudocentrous  vertebrae,  by  which  is  meant  that  a  thin  shell  of 
bone  surrounds  the  notochord.  Two  types  of  these  animals  are 
distinguished,  one  in  which  the  form  was  evidently  much  like  our 
modern  newts.  They  were  broad-headed,  had  several  pairs  of  gills, 
at  least  in  the  young  (Fig.  106).  As  an  example  of  the  skull  structure 
of  the  group,  that  of  Branchiosaurus  (Fig.  107),  one  of  the  most  gen- 
eralized of  vertebrate  skulls,  is  shown.  The  other  type  of  this  order 
was  a  snake-like  form,  without  limbs,  evidently  a  precociously  senes- 
cent type  in  which  the  ribs  reach  about  halfway  round  the  body. 

Order  II.  Stegocephali  Temnospondyli. — The  vertebrae  are  composed 
of  three  separate  pieces,  two  dorsal  and  one  ventral.  These  animals 
had  rather  long  ribs  and  their  armor  was  chiefly  ventral.  Some  of  the 
types  were:  Chelydosaurus,  a  turtle-like  form;  Dissorophus}  a  sort  of 
"Batrachian  Armadillo;"  Archegosaurus,  a  thoroughly  terrestrial 
form  about  five  feet  in  length. 

Order  III.  Stegocephali  Stereospondyli. — The  three  components  of 
the  vertebra  unite  into  one  solid  amphiccelous  vertebra.  This  group 
has  been  given  the  name  of  Labyrinthodonta  on  account  of  their  much 
folded  teeth.  The  vertebrae  are  sometimes  one-headed,  sometimes 
two-headed.  The  limb-girdles  are  very  primitive  and  strikingly  re- 
semble those  of  crossopterygian  fishes.  The  foot  skeleton  is  extremely 
generalized.  It  now  seems  likely  that  this  is  the  group  of  early  Am- 
phibia that  is  most  nearly  related  to  the  crossopterygian  fishes.  Wat- 
son has  studied  the  skulls  of  the  Carboniferous  labyrinthodonts 


182 


VERTEBRATE  ZOOLOGY 


Loxomma  and  Pteroplax  and  has  pointed 
out  their  many  striking  resemblances  to 
those  of  the  Carboniferous  crossopterygian 
Megalichthys.  These  resemblances  are 
carried  out  in  so  many  finer  details  that 
one  cannot  escape  the  conviction  that  the 
two  groups  are  closely  related. 

It  may  be  said  in  concluding  this  very 
much  abbreviated  account  of  the  extinct 
Amphibia,  that  recent  discoveries  of  early 
land  vertebrates  of  the  Texas  and  New 
Mexico  Permian  by  Williston  and  his 
colleagues,  has  revealed  a  number  of 
genera  that  show  a  combination  of  am- 
phibian and  reptilian  characters.  Some- 
FIG  106.— Stegocephahan,  times  ft  js  difficult  to  decide  readily 

Branchwsaurus  amblystomus.       i_    ,1         ,1  ,    ^ 

(From  Eastman-Zittel.)  whether  the  creature  belongs  to  one  or 

the  other  group.     These  forms  are  evi- 


FIG.  107. — A,  dorsal  and  B,  ventral  views  of  the  cranium  of  Branchiosaurus 
salamandroides  (after  Fritsch).  C,  posterior  view  of  cranium  of  Ti  emalosaurus 
(after  Fraas).  Br,  branchial  arches;  C,  condyle;  Ep,  epiotic;  F,  frontal;  J,  jugal, 
L.  O,  lateral  occipital  (exoccipital) ;  M,  maxillary;  Nt  nasal;  No,  nostril;  Pa, 
parietal;  PI,  palatine;  Pm,  premaxillary;  P.  o,  postorbital;  Pr.f,  prefrontal;  Ps, 
parasphenoid;  Pt,  pterygoid;  Ptf,  postfrontal;  O,  quadrate;  Oj,  quadrate  jugal; 
So,  supraoccipital;  Sq,  squamasal;  Si,  supratemporal;  V,  vomer.  (From  Gadow.) 


AMPHIBIA  183 

dently  Amphibia  that  have  developed  certain  dry-land  adaptations. 
Good  examples  of  this  transitional  group  are  Cacops  (Fig.  105,  C), 
Eryops  (Fig.  105,  F),  and  Cricotus  (Fig.  105,  D). 

The  Permian  was  the  period  in  which  the  amphibians  passed  their 
climax.  From  that  time  on  the  Amphibia  have  lived  a  hard  life  in 
competition  with  the  Reptilia  which  are  better  adapted  for  land 
life,  and  with  the  fishes  which  are  better  adapted  for  the  waters. 
Only  a  few  rather  small  groups  have  survived  and  these  largely 
through  their  retiring  habits  and  inconspicuous  appearance.  The 
group  of  Anura  has  recently  gained  a  secondary  dominance  through 
a  remarkable  adaptive  radiation  into  various  land  habitats. 

PRESENT-DAY  AMPHIBIA 
SUB-CLASS  II.     Liss AMPHIBIA  &- 

The  recent  Amphibia  are  the  Csecilia  or  Gymnophiona,  newts  and 
salamanders,  frogs  and  toads.  The  Amphibia  are  the  least  numerous 
of  the  vertebrate  classes,  except  the  Cyclostomata.  In  all  there  are 
only  about  1,000  species  (nearly  900  of  which  are  frogs  and  toads). 
This  is  to  be  compared  with  the  nearly  10,000  species  of  birds,  nearly 
8,000  species  of  fishes,  about  3,500  species  of  reptiles,  and  about  2,700 
mammals.  As  a  class  the  Amphibia  have  always  been  relatively 
unimportant  numerically,  possibly  because  it  is  essentially  an  "  in- 
between"  group,  as  has  been  shown. 

THE  CHAKACTERS  OF  THE  AMPHIBIA  (AFTER  GADOW) 

1.  The  vertebrae  are  (a)  acentrous,  Kj\  pseudocentrous,  or  (<^noto- 
— • —      centrous. 

2.  The  skull  articulates  with  the  atlas  by  two  condyles  which  are 
" — ^formed  by  the  lateral  occipitals  (exoccipitals). 

3.  There  is  an  auditory  columellar  apparatus  fitting  into  the  fenestra 

ovalis. 

4.  The  limbs  are  of  the  tetrapodous,  pentadactyle  type. 

__ie  red-blood  corpuscles  are  nucleated,  bjconyex,  and  ovaL^ 

6.  The  heart  is  (a)  divided  into  two  afna? (auricles)  and  one  ventricle, 

and  (b)  it  has  a  conus  provided  with  valves. 

7.  The  aortic  arches  are  strictly  symmetrical.' 

8.  Gills  are  present  at  least  during  some  early  stage  of  development 

9.  The  kidneys  are  provided  with  persistent  nephrpstomes. 


184  VERTEBRATE  ZOOLOGY 

10.  Lateral  line  sense  organs  are  present  at  least  during  the  larval 

stage. 

The  vagus  is  the  last  cranial  nerve. 

he  median  fins,  where  present,  are  not  supported  by  spinal 
skeletal  rays. 

13.  Sternal  ribs  and  a  costal  or  true  sternum  are  absent. 

14.  There  is  no  paired  or  unpaired  medio-ventral  copulatory  ap- 

paratus. 

15.  Development  takes  place  without  amnion  and  allantois. 
None  of  these  characters  is  absolutely  diagnostic,  except  1  (c),  and 

this  applies  only  to  Anura  and  most  of  the  Stegocephali. 

Numbers  1  (b),  1  (c),  2,  3,  4,  and  12  separate  the  Amphibia 
from  the  Fishes. 

Numbers  1,  6  (b),  7,  8,  9,  11,  13,  15,  separate  them  from  the  Rep- 
tiles, Birds,  and  Mammals. 

Number  2  separates  them  from  Fishes,  Reptiles,  and  Birds. 

Number  6  (a)  separates  them  from  the  Fishes  (excl.  Dipnoi), 
Birds,  and  Mammals. 

Gadow  says:  " Amphicondylous  Anamnia  would  be  an  absolutely 
correct  and  all-sufficient  diagnosis"  of  Amphiba,  but  concludes  that 
"Amphicondylous  animals  without  an  intra-cranial  hypoglossal 
nerve,"  is  a  more  practical  diagnosis. 

ORDER  I.  APODA  (GYMNOPHIONA) — LIMBLESS  AMPHIBIA 

The  Apoda  or  csecilians,  sometimes  called  "blind  worms,"  consti- 
tute a  small  group  of  about  forty  species,  living  in  the  warmer  parts 
of  the  world,  but  widely  distributed.  They  are  worm-shaped,  bur- 
rowing creatures  (Fig.  108,  A)  with  habits  somewhat  like  those  of 
earthworms  and  not  unlike  them  in  appearance.  They  have  no  limbs 
nor  limb-girdles  and  there  is  also  the  merest  rudiment  of  a  tail;  hence 
the  anal  opening  appears  to  be  terminal.  The  skin  is  folded  into  nu- 
merous ring-like  folds  and  is  smooth  and  slimy.  Small  deep-set  der- 
mal scales  occur,  which  are  believed  to  be  an  inheritance  from  stego- 
cephalian  ancestors.  The  cranium  is  very  solid  and  compact  in 
appearance,  more  like  that  of  a  reptile  than  that  of  other  modern  Am- 
phibia, but  the  same  bones  as  in  other  Amphibia  are  present  in  a 
broadened-out  form.  The  vertebrae  are  pseudocentrous  and  extremely 
numerous,  being  as  many  as  200  to  300  in  some  species.  The  eyes 
are  rudimentary  and  practically  functionless.  They  feel  their  way 


AMPHIBIA 


185 


about  by  means  of  a  protrusible  tentacular  organ  that  lies  in  a  groove 
between  the  eye  and  nose.  The  eggs  are  meroblastic  and  are  fertilized 
internally  by  means  of  an  eversion  of  the  cloaca  of  the  male  which  be- 
comes a  tube-like  copulatory  organ.  Some  species  are  oviparous, 
others  viviparous. 


FIG.  1^— -Group  of  Apoda.  A,  Caecilia,  emerging  from  burrow;  B,  Ichlhyo- 
phis  glutirw$y,s  (nat.  size),  female  guarding  her  eggs,  coiled  up  in  hole  in  the 
ground;  C,  a  nearly  ripe  embryo,  with  cutaneous  gills,  tiil-fih,  and  still  a  con- 
siderable amount  of  yolk.  (Redrawn  after  P.  and  F.  Sarasin.) 

Natural  History  of  Ichthyophis  glutinosa. — This  species  is  chosen 
as  an  example  of  Apoda  because  it  has  been  adequately  studied  and 
described  by  the  Sarasins.  The  species  extends  from  the  foot  hills 
of  the  Himalayas  to  Ceylon,  the  Malay  Archipelago,  and  Siam.  It 
reaches  a  length  of  about  a  foot.  In  color  it  is  dark  brown  or  bluish 
black  with  a  yellow  band  along  the  side.  The  ovarian  egg  is  oval, 
about  4x6  mm.  •  There  is  a  heavy  coat  of  albumen  with  chalazae, 
much  as  in  the  birds,  these  chalazae  uniting  the  eggs  in  bunches.  The 
egg  bunch  is  laid  in  a  shallow  hole  near  the  water.  The  female  coils 
herself  about  the  glutinous  mass  (Fig.  108,  B)  to  protect  it  from 
ground-burrowing  animals.  The  gilled  larval  period  is  passed  through 
in  the  egg  before  hatching.  The  three  pairs  of  larval  gills  (Fig.  108, 
C)  are  of  the  external  type  and  are  very  large  and  finely  branched. 
The  gills  are  lost  when  the  larva  hatches.  The  larva  swims  about  in 
the  water  for  a  time  like  an  eel,  but  comes  frequently  to  the  surface  to 
breath  air.  The  larval  period  is  a  long  one,  but  at  length  the  two 


186  VERTEBRATE  ZOOLOGY 

gill-clefts  close,  the  skin  changes  its  character,  the  tail-fin  disappears, 
and  it  emerges  upon  the  land  and  lives  a  burrowing  life.  So  exclu- 
sively terrestrial  does  it  become  that  it  drowns  if  after  metamorphosis 
it  is  put  in  water  for  any  length  of  time.  Several  other  genera  of 
Apoda  are  viviparous,  the  embryos  becoming  several  inches  in  length 
before  birth. 

The  Apoda  constitutes  a  very  degenerate  group.  In  some  respects 
they  are  more  primitive  than  other  living  Amphibia,  but  life  in  bur- 
rows has  caused  a  profound  degeneration  of  structure.  They  are  to 
be  included  among  the  eel-like  type  of  senescent,  degenerate  forms. 

ORDER  II.  URODELA — (TAILED  AMPHIBIA) 

This  order  is  represented  by  about  100  species  of  mud-puppies, 
salamanders,  newts,  and  efts.  They  range  in  habitat  from  forms  liv- 
ing permanently  in  the  water  and  breathing  with  external  gills  in  ad- 
dition to  lungs,  to  forms  that  live  after  metamorphosis  entirely  on 
land,  favoring  moist  woods  or  other  sheltered  places.  Some  authors 
group  all  forms  with  permanent  external  gills  in  a  separate  family, 
Perennibranchiata;  but  this  arrangement  is  believed  by  the  best  author- 
ities to  be  artificial  in  that  the  retention  of  the  aquatic  habit  and  lar- 
val gills  is  probably  due  to  arrested  development  and  may  have  taken 
place  in  more  than  one  family.  The  classification  given  here  is  taken 
from  Gadow  and  is  based  on  fundamental  anatomical  characters. 

Family  I.  Amphiumidce. — Without  gills  in  the  definitive  stage; 
gill-clefts  vestigial,  consisting  of  one  pair  of  small  openings,  or  en- 
tirely absent;  maxillary  bones  present;  teeth  on  both  jaws;  verte- 
brae amphiccelous;  both  fore  and  hind  limbs  present,  but  small; 
small  eyes  without  lids. 

The  family  is  represented  by  two  genera,  Cryptobranchus  and  Am- 
phiuma.  Cryptobranchus  allegheniensis  (Fig.  109,  A)  occurs  in  the 
mountain  streams  of  our  Eastern  States.  Another  species,  C.  japon- 
icus,  is  the  giant  salamander  of  Japan.  There  is  only  one  species  of 
Amphiuma  (Fig.  109,  B),  which  is  also  an  American  species  confined 
to  the  southeastern  States,  from  Carolina  to  Mississippi. 

Cryptobranchus  allegheniensis,  the  "hellbender,"  is  a  comparatively 
large  salamander  reaching  a  length  of  nearly  two  feet.  An  account 
of  the  development  of  this  species  and  certain  excellent  descriptions 
of  the  larvae  and  the  process  of  metamorphosis  have  been  furnished 
by  B.  G.  Smith.  The  eggs  are  fertilized  externally,  males  emitting 


AMPHIBIA 


187 


sperm  masses  near  the  egg-laying  female.  Batches  of  eggs  were  found 
in  the  shallow  parts  of  a  rather  large  stream  lying  on  the  gravelly 
bottom.  They  were  arranged  in  festoon-like  strings.  A  single  female 
lays  from  300  to  400  eggs.  The  cleavage  of  the  egg  is  especially  clean- 
cut  and  illustrates  the  transition  between  holoblastic  and  meroblastic 
cleavage.  The  animal  is  a  voracious  feeder,  capturing  fish  in  consid- 
erable numbers,  and  is  therefore  unpopular  with  fishermen.  The  larvae 
are  much  like  the  adults  of  Nedurus.  C.  japonicus,  is  very  much  like 


B 

allegheniensis; 


B,  Amphiuma  means.      (After 


FIG.    109.  —  A,  Cryptobranchus 
Lydekker.) 

C.  allegheniensis  in  appearance  and  in  habitat,  but  reaches  a  large 
size,  the  largest  specimens  being  about  five  feet  three  inches  in  length. 
This  is  the  extreme  size  reached  by  modern  Amphibia,  a  size  which 
almost  rivals  that  of  some  of  the  giant  land  Amphibia  that  became 
extinct  during  the  Permian.  In  Japan  these  animals  are  used  for  food 
and  are  caught  with  a  baited  hook,  the  hook  being  thrust  into  the 
retreat  of  the  animal  by  means  of  a  pole,  which  is  not  attached  to  the 
line,  and  may  be  removed  when  the  animal  seizes  the  bait. 

Amphiuma  means  (Fig.  109,  B)  is  an  eel-shaped  salamander  with 
limbs  very  much  reduced,  both  in  size  and  in  numbers  of  digits  (2  or 
3  being  the  characteristic  number).  One  pair  of  small  inconspicuous 
gill-clefts  is  present,  guarded  by  skin  flaps.  They  reach  a  length  of 


188 


VERTEBRATE  ZOOLOGY 


about  three  feet,  live  in  swamps  and  muddy  water,  often  invading 
the  rice  fields  of  the  Mississippi  lowlands.  The  rather  hard-shelled 
eggs  are  laid  in  festoons  and  are  protected  by  the  female,  which  lies 
about  them  in  a  coil.  The  larvae  have  well-developed  external  gills 
and  legs  relatively  larger  than  those  of  the  adult. 

Family   2.     Salamandridce.      (Salamanders   and   Newts.)      These 
urodeles  are  without  gills  in  the  adult  stage;  maxillaries  are  present; 

teeth  occur  in  both  jaws; 
eyes  have  movable  eyelids; 
fore  and  hind  limbs  present, 
but  sometimes  much  reduced. 
Nearly  three-fourths  of  the 
tailed  Amphibia  belong  to 
this  family.  Only  a  few  typi- 
cal species  can  be  mentioned 
here. 

Desmognathus  fuscus   (Fig. 
FIG.  nO.-Desmognathus  fuscus;    female    no)  jg  Qne  of  Qur  commonest 
with  eggs  in  hole  underground,    (trom  ua-  .  _    . 

dow,  after  Wilder.)  American  newts.    It  is  a  small 

type,  about  four  inches 
length,  living  a  noctur- 
nal life,  hiding  in  the 
daytime  under  stones  or 
concealed  along  the  edges 
of  mountain  streams. 
The  color  is  brown  suf- 
fused with  pink  and  gray. 
They  are,  strange  to  say, 
lungless,  the  process  of 
respiration  being  carried 
on  in  the  skin  and  pos- 
sibly also  by  the  mucous  FlQ  nl._Spelerpes  ffu8CU8f  showing  the  posi. 
lining  of  the  intestine,  tion  and  shape  of  the  partly  protruded  tongue 
The  eggs  are  laid  in  a  and  tne  tongue  skeleton  on  the  right.  T,  tongue; 
,  i  '  i  B,  branchial  arch;  7/,  hyoid.  (From  Gadow,  after 

bunch,  each  egg  attached   B;rg  and  wieders'heim.) 

by  a  string,   the  whole 

group  looking  like  a  bunch  of  toy  balloons.  The  female  lays  the 
eggs  in  a  hole  in  the  mud  and  coils  her  body  partly  about  them. 
After  a  period  of  incubation  the  eggs  hatch  and  give  forth  larvse 


AMPHIBIA  189 

which  are  nearly  definitive  in  form.  Spelerpes  bilineatus,  another 
newt  of  the  Atlantic  States,  has  been  described  by  H.  H.  Wilder. 
It  is  about  four  inches  in  length,  brownish  yellow  in  color  above, 
with  a  black  lateral  line,  and  brightest  yellow  beneath.  It  lives 
a  nocturnal  life  and  hides  under  stones  or  logs.  The  eggs  are 
laid  in  bunches  of  thirty  to  fifty  on  the  under  sides  of  submerged 
stones.  S.  fuscus  (Fig.  Ill)  has  an  extremely  extensible  tongue,  capa- 
ble of  being  shot  out  nearly  two  inches.  With  this  it  captures  in- 
sects by  means  of  a  sticky  disk  on  its  end. 

Amblystoma  tigrinum  (Fig.  112,  A)  the  "tiger  salamander"  is  the 
commonest  and  most  widely  distributed  of  North  American  salaman- 
ders. It  occurs  from  the  Atlantic  Ocean  to  Minnesota  and  well  into 
the  Southern  States.  The  ground  color  is  nearly  black,  with  large  yel- 
low spots  and  blotches,  which  sometimes  merge  into  broad  stripes  or 
bands.  It  lives  in  damp  situations  on  land,  under  stones  or  logs,  and 
is  not  infrequently  found  in  cellars.  The  large  prominent  golden 
eyes,  the  very  broad  head  and  large  mouth,  are  characteristic  features. 
The  length  varies  from  five  to  nine  inches.  This  species  is  best  known 
because  of  the  fact  that  it  exhibits  a  classic  case  of  pcedogenesis  or  neo- 
ieny,  the  capacity  to  become  sexually  mature  while  still  retaining  the 
larval  body.  The  larva  of  Amblystoma  is  the  classic  Axolotl  (Fig.  112, 
B),  which  has  for  a  long  time  been  abundant  in  the  lakeonear  Mexico 
City.  It  was  supposed  to  be  a  perennibranchiate  species  and  was 
called  "Siredon  axolotl."  The  true  situation,  however,  was  revealed 
when  some 'of  these  larvae  were  kept  in  aquaria  in  Paris.  Some  of 
them  lost  their  gills  and  other  aquatic  adaptations  and  metamorphosed 
into  the  well-known  Amblystoma  tigrinum,  a  purely  terrestrial  type. 
It'  was  found  possible  by  experimental  means  to  control  the  metamor- 
phosis so  as  to  keep  the  animals  permanently  as  Axolotls,  or  to  cause 
them  to  metamorphose  promptly  into  adult  Amblystomas.  Even 
after  metamorphosis  had  begun  it  was  possible  to  check  it  and  cause 
the  animals  to  revert  to  the  Axolotl  condition.  This  situation  throws 
light  on  the  significance  of  some  of  the  perennibranchiate  species 
that  never  metamorphose;  for  it  has  often  been  suggested  that  these 
forms  are  permanent  larvae  or  that  they  exhibit  a  type  of  racial 
psedogenesis  that  has  become  so  fixed  that  metamorphosis  is  no 
longer  possible. 

Salamandra  maculosa,  the  " spotted"  or  "fire  salamander"  is  one 
of  the  commonest  salamanders  of  Europe,  having  a  wide  range  over 


190 


VERTEBRATE  ZOOLOGY 


the  whole  of  central,  southern,  and  western  Europe.  It  lives  under 
moss  or  rotten  leaves,  in  cracks  in  the  ground  or  in  the  roots  of  old 
trees;  in  fact,  in  almost  any  moist  place.  It  is  about  the  same  size 


FIG.  112. — Group  of  Urodela.  A,  Salamandra  maculosa  (after  Lydekker.) 
B,  Axolotl  larva  of  Amblystoma  tigrinum;  C,  female  and  D,  male  of  Triton 
cristatus  (male  in  nuptial  dress)  x  2/3.  (After  Gadow.) 

as  Amblystoma  and  resembles  it  in  form  and  habit.  The  color  pattern 
is  also  similar,  with  its  black  groundwork  and  yellow  spots.  Fire 
salamanders  are  poisonous,  as  is  shown  by  the  quick  death  of  bull- 
frogs, snakes,  or  warm-blooded  animals  that  have  eaten  them;  the 


AMPHIBIA  19] 

poison  is  due  to  a  cutaneous  secretion.  The  breeding  habits  are 
rather  odd.  In  July  after  a  preliminary  exciting-  performance  between 
the  sexes  on  land,  both  males  and  females  go  into  the  water,  but  leave 
the  heads  out.  The  male  deposits  a  spermatophore,  or  package  of 
sperm,  which  the  female  partly  takes  into  the  cloaca.  Fertilization 
is  internal  and  slow  development  occurs  in  the  uterus,  taking  about 
ten  months  to  complete  itself.  The  well-developed  young,  to  the 
number  of  about  fifteen  or  so,  are  born  in  the  water.  The  species  is 
therefore  truly  viviparous.  The  larvae  have  external  gills  and  live  in 
the  water  for  about  four  months  and  then  very  slowly  metamorphose 
into  the  terrestrial  adult  form. 

Salamandra  atra  is  an  alpine  form  like  S.  maculosa  but  much  darker. 
It  occurs  in  mountain  lakes  at  an  altitude  of  2,000  to  9,000  feet  above 
sea  level.  It  produces  only  two  young  at  a  birth.  These,  while  still 
in  the  uterus,  feed  upon  the  other  eggs  found  there  and  metamorphose 
completely  before  birth.  Kammerer  claims  that  S.  atra  can  be 
changed  into  S.  maculosa  by  bringing  them  into  the  lowland  waters 
and  that  after  they  have  been  kept  there  for  a  few  generations  they 
tend  to  retain  the  breeding  habits  of  the  lowland  form  though  trans- 
ferred back  to  the  Alpine  environment.  This  has  often  been  cited 
as  evidence  in  favor  of  the  inheritance  of  acquired  characters,  a  doc- 
trine which  is  quite  generally  unacceptable  to  biologists,  but  is 
strongly  advocated  by  a  small  but  growing  minority. 

Diemictylus  viridescens  is  a  good  example  of  the  "efts,"  sometimes 
also  called  "newts."  It  is  commonly  called  the  "vermilion  spotted 
eft."  It  has  a  prolonged  life  history,  taking  several  years  to  reach 
full  maturity.  For  the  first  three  years  it  lives  in  the  water,  being 
green  in  color  and  having  external  gills.  It  then  leaves  the  water  and 
becomes  yellow  with  vermilion  spots.  After  some  time  it  again  re- 
turns to  the  water,  becomes  green,  and  lives  an  aquatic  life  during  the 
breeding  season,  after  which  it  once  more  takes  on  the  terrestrial 
features  and  migrates  to  land.  The  life  cycle  of  this  species  illustrates 
as  well  as  any  other  the  extreme  plasticity  of  the  group  and  the  deli- 
cate equilibrium  that  exists  between  the  aquatic  and  terrestrial 
phases. 

Triton  cristatus  (Fig.  112,  C  and  D),  the  "crested  newt,"  received  its 
name  from  the  fact  that  in  the  male  (Fig.  112,  D)  there  is  a  pronounced 
dorsal  crest  during  the  breeding  or  nuptial  season.  The  color  at  this 
time  is  also  very  striking,  the  top  of  head  being  marbled  black  and 


192  VERTEBRATE  ZOOLOGY 

white,  the  under  side  yellow  with  black  spots,  and  the  side  of  the  tail 
has  a  broad  bluish-white  band.  The  female  has  quieter  colors,  a  gen- 
eral brownish-black  ground  with  a  yellow  line  down  the  middle  of  the 
back.  This  is  the  most  pronounced  instance  of  sexual  dimorphism 
among  the  Urodela  and  possibly  within  the  class  Amphibia.  The 
crested  newt  has  a  wide  distribution,  occurring  in  England  and  Scot- 
land and  through  Central  Europe.  Many  other  species  of  the  genus 
Triton  occur,  two  of  which,  T.  torosus  and  T.  virescens,  occur  in  North 
America,  the  latter  being  common  through  the  Eastern  United  States. 

Family  3.  Proteidce.  The  Mud-Puppies. — These  animals  have 
three  pairs  of  fringed  external  gills  throughout  life  (perennibranchi- 
ate);  both  fore  and  hind  limbs  are  present;  eyes  are  without  lids; 
maxillaries  are  absent;  teeth  occur  on  premaxillaries,  vomers,  and  man- 
dible; vertebra  are  amphiccelous.  Only  three  genera,  each  repre- 
sented by  a  single  species,  occur,  two  in  America  and  one  in  Europe. 

Necturus  maculatus  (113,  A)  is  the  common  American  "mud-puppy," 
so  called  because  the  fringed  gills  look  something  like  the  pendant 
hairy  ears  of  a  water  spaniel.  They  are  found  all  over  the  eastern 
part  of  the  United  States  and  in  eastern  Canada.  They  are  about  a 
foot  in  length,  of  a  muddy-brown  color,  mottled  with  blackish  spots. 
Behind  the  dark  red  external  gills  are  paired  gill-clefts.  These  am- 
phibians impress  one  as  rather  dull,  stupid  animals,  living  a  sluggish 
life  on  the'  muddy  bottoms  of  lakes  and  rivers.  At  times,  however, 
they  move  about  quite  smartly  with  graceful  eel-like  motion.  They 
are  active  chiefly  at  night,  when  they  swim  about  in  search  of  frogs, 
Crustacea,  worms,  fishes,  and  insects.  They  are  often  found  in  very 
cold  water  and  seem  to  be  well  adapted  to  low  temperatures.  In 
general  it  may  be  said  that  these  animals  resemble  closely  the  Iarva3 
of  the  Salamandridse,  especially  that  of  Amblystoma  (Axolotl); 
a  fact  that  has  given  rise  to  the  idea  that  they  are  not  truly  primitive 
aquatic  Amphibia  at  all,  but  simply  a  species  that,  after,  possibly 
thousands  of  years  of  psedogenetic  habit,  has  lost  its  plasticity  and  is 
no  longer  able  to  metamorphose  from  the  larval  to  the  adult  condi- 
tion. It  would  be  interesting  to  try  thyroid  feeding  experiments 
upon  the  larvae  with  the  idea  of  inducing  metamorphosis  into  a  true 
adult  type. 

Proteus  anguineus  (Fig.  113,  B),  "olms"  as  the  Germans  call  them, 
are  blind  cave  mud-puppies.  The  whole  body  is  white  or  nearly  so. 
If  kept  in  places  where  light  is  not  absolutely  excluded  they  become 


AMPHIBIA 


193 


at  first  grayish  and  later  jet-black.  The  eyes  are  rudimentary  and 
completely  hidden  beneath  the  opaque  skin.  It  has  been  suggested 
that  the  conditions  for  development  are  such  as  to  inhibit  certain 


FIG.  113. — Group  of  perennibranchiate  urodeles.  A,  Necturus  maculatus;  B, 
Proteus  anguineus;  C,  Typhlomolge  rathbuni;  D,  Siren  lacertina.  (Redrawn  after 
Lydekker  and  others.) 

structures  such  as  the  eyes  and  pigment  from  developing.  Possibly 
this  might  also  be  tied  up  with  the  permanently  larval  condition. 
They  require  very  little  nourishment,  probably  being  accustomed  to 
only  a  minimum  amount  in  their  natural  habitat. 


194  VERTEBRATE  ZOOLOGY 

Typhlomolge  rathbuni  (Fig.  113,  C)  a  form  very  much  like  Proteus,  is 
a  native  of  Texas.  It  inhabits  subterranean  caves  and  is  sometimes 
brought  to  the  surface  in  water  from  artesian  wells.  It  is  likely  that 
both  of  these  cave  mud-puppies  arose  independently,  in  response  to 
similar  conditions,  from  some  form  like  Necturus. 

Family  4-  Sirenidce.  (The  Sirens.) — They  have  three  pairs  of  per- 
manent fringed  external  gills;  the  body  is  eel-like,  and  there  are  no 
hind  limbs;  maxillaries  are  absent;  no  teeth  are  present,  except  some 
small  ones  on  the  vomer;  jaws  are  furnished  with  a  horny  sheath  like 
that  of  frog  larvae;  there  are  no  eye-lids.  There  are  two  genera,  each 
represented  by  a  single  species. 

Siren  lacertina  (Fig.  113,  D),the  "mud-eel,"  has  three  pairs  of  gill- 
clefts  and  external  gills  in  the  adult.  It  reaches  a  length  of  two  and 
a  half  feet.  The  tail  is  strongly  compressed  and  with  well-developed 
fins.  The  color  is  blackish  above  and  lighter  below.  The  animal  lives 
in  the  mud  at  the  bottom  of  ponds.  It  is  found  in  the  southeastern 
parts  of  the  United  States. 

Pseudobranchus  striatus  is  very  much  like  Siren  but  is  smaller,  sel- 
dom exceeding  seven  inches  in  length.  It  has  only  one  pair  of  gill- 
clefts  and  only  three  fingers.  A  broad  yellow  band  along  the  side  re- 
lieves the  somber  coloration. 

The  Sirenidae  are  considered  the  most  degraded  of  urodeles.  That 
they  are  not  truly  primitive  is  borne  out  by  the  observation  of  Cope 
that  the  young  lose  the  external  gills  which  then  redevelop  in  the 
adult;  so  the  adult  condition  is  a  renewed  or  secondary  larval  con- 
dition. This  and  other  observations  tend  to  confirm  the  theory  of 
psedogenesis  as  applied  to  both  Serenidse  and  Proteidse. 

Which  of  the  Anura  are  to  be  considered  the  most  primitive,  the 
nearest  approach  to  the  first  ancestral  Amphibia?  It  seems  certain 
that  the  perennibranchiate  forms  are  not  ancestral  but  merely  re- 
tain, or  return  to,  a  larval  condition.  Probably  Cryptobranchus  rep- 
resents a  condition  more  nearly  primitive  or  ancestral  than  any 
other  living  amphibian. 

PSEDOGENESIS   OR   NEOTENY 

There  is  perhaps  no  better  group  of  vertebrates  for  illustrating  the 
phenomenon  of  psedogenesis  or  the  retention  of  larval  structures 
during  sexual  maturity.  It  is  a  common  phenomenon  in  a  number 
of  groups  of  invertebrates  and  especially  so  among  the  most  highly 


AMPHIBIA  195 

specialized  orders.  In  the  Diptera,  for  example,  there  are  several 
species  in  which  young  larvae  or  pupae  produce  young.  The  genus 
Miastor  is  a  classic  case.  Here  the  young  while  still  within  the  mother 
become  sexually  mature. 

The  prolongation  of  larval  life  was  noted  for  the  Ammocoetes 
larva  of  the  lamprey,  which  requires  from  three  to  four  years  to  reach 
the  time  of  metamorphosis.  The  larva  is  much  like  Amphioxus,  and 
if  psedogenesis  should  occur  in  this  species  we  would  have  a  case  quite 
parallel,  I  believe,  to  that  seen  in  the  perennibranchiate  urodeles.  A 
permanent  Ammoccetes  would  be  classed  as  an  extremely  primitive 
chordate  not  very  distantly  related  to  Amphioxus;  in  fact,  before 
Ammoccetes  was  discovered  to  be  the  larva  of  Petromyzon,  it  was  so 
considered.  May  it  not  be  possible  that  Amphioxus  itself  is  a  perma- 
nent larva  of  some  cyclostome  more  primitive  than  the  myxinoids  or 
the  petromyzonts?  Possibly  it  is,  but  such  a  view  would  seem  to  dis- 
credit the  Amphioxus  theory  of  the  ancestry  of  the  vertebrates,  a 
view  dear  to  our  hearts  and  one  that  we  should  be  reluctant  to  aban- 
don. 

The  causes  of  psedogenesis  are  obscure,  but  we  are  at  least  justified 
in  attributing  the  foreshortening  of  development  of  the  soma  to  cer- 
tain growth-inhibiting  agents  or  to  the  absence  of  certain  stimuli  to 
metamorphosis.  In  general  it  may  be  said  that  low  temperatures  re- 
tard development  and  produce  defective  organisms;  and  the  perenni- 
branchiate Amphibia  live  in  water  that  is  low  in  temperature.  Pos- 
sibly these  animals  are  merely  senescent  and  have  lost  so  much 
growth  momentum  that  they  are  unable  to  push  through  to  com- 
plete adult  differentiation. 

Certain  experiments  on  anuran  larvae  seem  to  throw  some  light 
on  this  matter.  It  has  been  shown  by  several  writers,  in  the  case  of 
certain  species  of  frog  which  have  a  prolonged  larval  period,  that 
prompt  metamorphosis  may  be  induced  by  thyroid  feeding.  It  is  also 
possible  to  induce  precocious  metamorphosis  in  other  species  by  sim- 
ilar methods.  This  suggests  that  the  underlying  cause  of  paedogenesis 
may  have  something  to  do  with  the  failure  of  the  thyroid  to  function 
or  to  a  deficiency  in  its  secretion.  The  recent  experiments  of  B.  W. 
Allen  and  his  pupils  are  of  interest  in  this  connection.  It  was  shown 
that  the  early  extirpation  of  the  hypophysis  in  tadpoles  prevents  the 
development  of  the  thyroid  gland  and  that  the  operated  individuals 
remain  permanent  larvae. 


196  VERTEBRATE  ZOOLOGY 

ORDER  III.  ANURA  (TAILLESS  AMPHIBIA) 

The  frogs  and  toads  are  the  characteristic  Amphibia  of  the  present 
age.  They  are  represented  by  about  900  species  and  exhibit  a  very 
pronounced  adaptive  radiation.  They  are  the  most  highly  specialized 
of  modern  Amphibia  and  so  much  specialization  exists  within  the 
order  that  there  is  difficulty  in  listing  characters  that  apply  to  all  of 
its  members.  Not  only  has  there  been  adaptive  radiation  in  the  order, 
but  all  of  the  large  families  exhibit  a  radiation  into  terrestrial,  arboreal, 
aquatic,  and  burrowing  types.  Since  the  frog  is  a  favorite  type  ver- 
tebrate and  is  used  in  nearly  all  elementary  courses  in  zoology,  it  will 
save  time  and  space  to  omit  any  detailed  anatomical  description  of  a 
type  form.  We  shall  therefore  proceed  to  give  an  abbreviated  sys- 
tematic survey  of  the  various  groups  of  Anura.  Two  suborders  are 
distinguished:  the  Aglossa,  which  are  without  a  tongue  and  have 
eustachian  tubes  united  in  one  pharyngeal  opening;  and  the  Phaner- 
oglossa,  in  which  a  tongue  is  present.  The  first  is  a  coherent  natural 
group,  but  the  second  may  be  an  artificial  assemblage. 

SUB-ORDER  I.  AGLOSSA 

This  is  a  small  group  of  frogs  not  well  known  to  the  layman.  Three 
genera,  Pipa,  Xenopus,  and  Hymenochirus  occur.  Of  these  Pipa  is  a 
South  American  tropical  form  and  the  other  two  belong  to  Africa. 

Pipa  americana  (the  Surinam  toad)  is  a  classic  object  to  the  zoolo- 
gist on  account  of  its  unique  breeding  habits  (Fig.  114,  A).  The  crea- 
ture is  an  odd,  ugly  aquatic  toad,  with  exceedingly  large  hind  feet  and 
a  very  short,  broad  head.  The  following  description  of  its  spawning 
is  described  by  Bartlett: — "About  the  28th  of  April  the  males  became 
very  active  and  were  constantly  heard  uttering  their  most  remarkable 
metallic  call-notes.  On  examination  we  then  observed  two  of  the 
males  clasping  tightly  around  the  lower  part  of  the  bodies  of  the 
females,  the  hind  parts  of  the  males  extending  beyond  those  of  the 
females.  On  the  following  morning  the  keeper  arrived  in  time  to 
witness  the  mode  in  which  the  eggs  were  deposited.  The  oviduct  of 
the  female  protruded  from  the  body  more  than  an  inch  in  length,  and 
the  bladder-like  protrusion  being  retroverted,  passed  under  the  belly 
of  the  male  on  to  her  own  back.  The  male  appeared  to  press  tightly 
upon  the  protruded  bag  and  to  squeeze  it  from  side  to  side,  apparently 
pressing  the  eggs  forward  one  by  one  on  to  the  back  of  the  female.  By 


AMPHIBIA 


197 


this  movement  the  eggs  were  spread  with  nearly  uniform  smoothness 
over  the  whole  surface  of  the  back  of  the  female  to  which  they  became 


FIG.  114. — Frogs  and  Toads  (Anura)  I.  A,  Surinam  Toad,  Pipa  americana; 
B,  Fire-bellied  toad,  Bombinator  igneus;  C,  Midwife  Toad,  Alytes  obstetricans; 
D,  Spade-foot  Toad,  Pelobates  cultripes;  E,  foot  of  Pelobates  showing  tarsal  spur; 
F,  Common  Toad,  Biifo  lentigwosus  s.  americanus,  with  vocal  sac  inflated.  G, 
same  stalking  its  prey.  (A  and  G,  redrawn  after  Lydekker;  B,  D,  and  E,  redrawn 
after  Gadow;  F  and  G,  redrawn  after  Dickerson.) 


198  VERTEBRATE  ZOOLOGY 

firmly  adherent."  The  eggs  then  sink  into  pockets  in  the  skin.  Each 
pocket  develops  a  sort  of  hinged  lid,  which  the  young  toad  pushes 
open  from  time  to  time,  as  in  Fig.  1 14,  A.  The  habits  of  the  African 
Aglossa  are  less  specialized  and  have  no  features  of  especial  interest 
for  the  general  student. 

SUB-ORDER  2.     PHANEROGLOSSA  (TONGUED  ANURA) 

There  are  seven  families  of  these  frogs  and  toads  and  only  the  most 
general  distinguishing  characters  of  these  groups  can  be  given.  As  in 
any  other  group  that  has  undergone  pronounced  adaptive  radiation, 
the  chief  points  that  interest  the  student  are  the  special  structural 
adaptations  and  peculiar  habits.  Under  each  family  only  the  best 
known  and  most  interesting  species  will  be  described. 

Family  1.  Discoglossidce. — The  tongue  is  disk-shaped  and  non- 
protrusible;  the  vertebrae  opisthocoelous;  the  upper  jaw  and  vomers 
have  teeth;  and  the  male  has  no  vocal  sac. 

Bombinator  igneus,  the  "fire-bellied  toad,"  is  a  poisonous  form  with 
pronounced  warning  coloration  and  a  special  method  of  displaying  it. 
The  under  surface  is  colored  a  purplish  black  with  conspicuous  orange- 
red  patches.  They  are  decidedly  aquatic,  floating  at  the  surface  with 
legs  extended  so  that  the  conspicuous  color  is  well  displayed  to  all 
aquatic  enemies.  They  also  rest  on  land  and  when  surprised  there 
they  make  a  strong  effort  to  bring  the  under  surface  to  view 
by  turning  the  legs  over  the  back  and  throwing  back  the  head  as  in 
Fig.  114,  B.  Besides  their  coloration  they  are  interesting  because  of 
the  weird  noises  they  make.  The  voice  is  described  as  like  "hoonk, 
hoonk"  or  "ooh,  ooh,"  and  the  males  join  in  a  mournful  concert  of 
sound  during  the  breeding  season. 

Alytes  obstetricans  (Fig.  114,  C),  the  "midwife-toad,"  is  in  general 
appearance  quite  ordinary.  It  occurs  in  France  and  Switzerland. 
The  interesting  feature  of  the  species  is  the  method  of  caring  for  the 
eggs  by  the  males  and  the  latter  ?s  odd  habit  of  relieving  the  female  of 
her  eggs.  The  male  assiduously  massages  the  cloaca  of  the  female 
with  the  paws.  After  a  considerable  time  the  female  suddenly  and 
with  great  apparent  effort  expels  the  eggs  all  in  a  bunch.  The  male 
then  clings  to  the  female 's  head  and  fecundates  the  eggs,  after  which 
he  carries  the  bunch  of  eggs  off  with  him,  attached  to  the  hind  legs, 
to  a  hole  in  the  ground.  He  moistens  the  eggs  with  dew  and  occasion- 
ally takes  them  into  the  water  with  him.  When  the  eggs  are  nearly 


AMPHIBIA  199 

ready  to  hatch  he  betakes  himself  to  the  water  for  the  period  of 
hatching. 

Family  2.  Pelobatidce. — The  tongue  is  oval,  hitched  in  front  but 
free  behind,  so  that  it  can  be  thrown  out;  upper  jaw  and  vomers  with 
teeth ;  vertebrae  procoelous.  There  are  seven  genera  consisting  of  about 
20  species.  Pelobates  cultripes  (Fig.  114,  D),  the  " spade-foot  toad/'  is 
the  best  known  of  the  family.  They  are  typically  burrowing  toads, 
digging  rather  deep  holes  in  sand  and  resting  there  during  the  day. 
The  "  spade"  is  a  modified  hind  foot,  which  has  a  strong  spur  (Fig.  114, 
E)  on  its  under  surface  which  aids  in  digging.  The  species  are  noc- 
turnal in  feeding  habits. 

Family  3.  Bufonidce.  The  Common  Toads. — They  have  no  teeth  in 
upper  or  lower  jaws;  vertebras  procoelous  and  without  ribs.  They 
are  for  the  most  part  decidedly  terrestrial,  some  of  them  occupying 
arid  territory. 

Bufo  vulgaris  (Fig.  114,  F  and  G),  the  common  toad  of  North  Amer- 
ica and  other  palearctic  regions,  is  typical.  The  color  is  variable  and 
changeable,  highly  protective  in  its  resemblance  to  the  background. 
They  are  nocturnal  in  habits,  feeding  on  worms,  insects,  and  snails. 
One  sees  them  frequently  under  electric  lights  waiting  for  dazed  in- 
sects to  drop  to  the  ground.  They  hop  quickly  to  the  fallen  insect  and 
snap  it  up  suddenly.  Earthworms  are  crushed  and  squeezed  till 
comparatively  quiet  before  swallowing  occurs.  In  the  daytime  toads 
hide  under  stones  or  in  dark  corners.  They  breed  in  temporary  pools 
in  the  early  spring.  The  newly  hatched  toads  are  surprisingly  small 
and  require  nearly  five  years  to  reach  maturity. 

Toads  are  almost  without  enemies  on  account  of  their  noxious  skin 
secretions.  About  the  only  agency  in  keeping  down  their  numbers 
appears  to  be  parasites  and  epidemics  of  disease. 

Family  4-  Hylidce.  (Tree-Frogs  or  Tree-Toads.) — Upper  jaw  and 
vomers  with  teeth,  lower  jaw  also  toothed  in  one  species  Amphig- 
nathodon;  vertebrae  proccelous  without  ribs;  fingers  armed  with  ad- 
hesive pads;  tongue  protrusible  to  varying  degrees.  They  are  all 
climbing  arboreal  frogs,  many,  but  not  all,  being  green  in  color.  They 
are  very  widely  distributed  and  have  in  all  about  150  species;  hence 
this  is  one  of  the  largest  families.  The  genus  Hyla  is  the  most  gener- 
alized and  wide-spread  genus.  H.  versicola  (Fig.  115,  A)  is  the  com- 
mon tree-toad  of  the  Northern  United  States.  It  emits  a  "  clear,  loud, 
thrilled  rattle"  quite  familiar  to  most  naturalists.  These  tree-toads 


200 


VERTEBRATE  ZOOLOGY 


D 


FIG.  115. — Frogs  and  Toads,  (Anura)  II.  A,  Tree-Toad,  Hyla  versicola;  B, 
Nototrema  marsupium  with  brood-pouch  laid  back  to  show  inclosed  eggs;  C,  Hyla 
arborea,  with  vocal  sac  expanded;  D,  Javan  Flying  Frog,  Rhacophorus  pardalis; 
E,  Leopard  Frog,  Rana  pipiens;  F,  Bull-Frog,  Rana  catesbiana.  G,  Rana 
esculenta,  showing  the  movement  of  the  tongue  in  capturing  a  fly.  (A,  E,  and  F, 
redrawn  after  Dickerson;  B  and  G,  after  Gadow;  C  and  D,  redrawn  after  Ley- 
decker.) 


AMPHIBIA  201 

change  color  quickly  from  dark  brown  to  a  delicate  gray.  In  the 
daytime  they  hide  quietly  in  sheltered  crevices  of  bark  or  the  crotches 
of  limbs,  but  at  night  they  become  lively  and  noisy,  jumping  about 
and  busily  catching  insects.  They  breed  in  shallow  pools  in  May, 
passing  through  a  regular  tadpole  stage,  and  metamorphose  into 
small  perfect  frogs  in  about  seven  weeks. 

Hyla  faber,  a  native  of  Brazil,  is  one  of  the  most  interesting  of  the 
tree-frogs,  on  account  of  its  remarkable  voice  and  extraordinary  nest- 
building  habits.  Its  voice  is  said  to  resemble  that  of  a  mallet  beaten 
against  a  copper  plate.  When  caught  it  utters  a  cry  like  that  of  a 
wounded  cat.  It  makes  an  aquarium-like  nest  for  its  eggs  and  young, 
by  digging  a  basin  of  some  depth  in  the  bottom  of  shallow  pools  and 
building  a  mud  wall  about  it.  The 
whole  inside  of  the  basin  is  most 
carefully  smoothed  off  by  rubbing 
the  belly  over  it.  The  male  takes 
no  part  in  this  building  opera- 
tion, but  raises  an  unearthly 

racket  all  the  while. 

T7  7  ,,..   /T71.       -,--,a\    •  FIG.   116. — Hyla    gcddii   xl.     Female 

Hyla   goddn  (Fig.  116)   is  re-  with  eggg  in   [JVjg*  dorgal  brood. 

markable  on  account  of  the  fact   pouch.     (From  Gadow.) 

that  the  female  carries  the  eggs 

on  the  back  in  a  shallow  depression  till  they  are  almost  ready 

for  metamorphosis. 

The  genus  Nototrema  (Fig.  115,  B)  differs  from  Hyla  in  that  the 
female  has  an  egg  pouch  or  marsupium  on  the  back,  which  is  merely 
a  fold  of  the  skin.  It  has  been  suggested  that  the  marsupium  may  be 
a  specialization  of  the  simple  pocket  seen  in  Hyla  gcddii. 

The  tiny  Acris  gryllus  (Fig.  115,  C),  or  "Cricket  Frog"  of  eastern 
and  central  United  States,  is  one  of  our  smallest  frogs.  It  is  described 
as  a  merry  little  frog,  chirping  constantly  even  in  captivity.  It  fre- 
quents the  borders  of  pools,  jumping  into  the  water  if  disturbed  and 
quickly  burying  itself  in  the  mud  at  the  bottom. 

Family  5.  Cystignathidce  is  another  of  the  large  anuran  families, 
having  like  the  Hylidse  about  150  species.  They  play  about  the 
same  role  in  the  southern  continents  (Notogaea)  as  the  Ranidse 
do  in  the  northern  continents  (Arctogsea).  Some  of  them  are 
difficult  to  distinguish  from  the  Ranidae.  The  whole  family  is 
ill-defined,  being  specialized  in  a  great  many  directions.  They 


202 


VERTEBRATE   ZOOLOGY 


sometimes  have  adhesive  pads  on  the  toes.  Only  one  genus  need 
be  mentioned. 

The  genus  Hylodes,  of  which  there  are  nearly  50  species,  is  one  of 
the  best  known.  They  are  tree-toads  much  like  those  of  the  genus 
Hyla.  Hylodes  martinicensis  is  a  tiny  frog  in  which  the  pairing  and 
egg  laying  take  place  on  land,  the  eggs  in  a  foamy  mass  being  glued 
to  a  leaf.  The  large  eggs,  about  4-5  mm.  in  diameter,  develop 

practically  to  metamorphosis  before 
hatching,  the  aquatic  larval  period 
being  omitted  (Fig.  117). 

Family  6.  Engystomatidce  (Narrow- 
Mouthed  Toads).  —  They  are  some- 
times called  "toothless  toads."  A 
representative  of  this  family  in  the 
United  States  is  Engystoma  caroli- 
nense,  a  family  common  in  the  old 
South.  They  are  sharply  distin- 
guished by  the  dilation  of  the  sacral 
diapophyses.  Perhaps  the  most  sig- 
nificant feature  of  the  family  has  to 
do  with  their  ant-eating  habits  and 

adaptations  for  it.    It  is  well  known 
FIG.  117.  —  Hylodes  martinicensis.    ,1,1  .  •        i     *  ..    . 

1,  an  egg  with  embryo  about  seven  that  the  ant-eating  habit  in  various 
days  old;  2,  another,  twelve  days  groups  of  vertebrates  is  associated 
old;  3,  the  young  frog  just  hatched;  wjtn  rather  definite  changes  in  struc- 

,  ,-,,  „  , 

ture'  These  fr°Ss  have  a  narrow 
mouth,  protruding  snout,  toothless 

condition,  hidden  tympanum,  modified  feet  and  shoulder  girdle  for 
digging;  these  are  also  characters  of  ant-eaters  in  other  vertebrate 
classes  such  as  Reptilia  and  Mammalia. 

Family  Ranidce.  —  These  are  the  "true  frogs"  and,  to  dwellers  of 
the  northern  hemisphere,  the  most  familiar  of  amphibians.  Two 
small  families,  one  consisting  of  one  species,  the  other  of  about  a 
dozen  species,  differ  from  the  Raninse  (the  typical  frogs)  chiefly  in 
the  teeth. 

Sub-Family  1.  Ceratobrachince,  with  teeth  in  upper  and  lower  jaws. 
Ceratobrachus  guentheri  is  a  native  of  the  Solomon  Islands. 

Sub-Family  2.  Dendrobatinoe.  —  Arboreal  frogs  of  small  size  without 
any  teeth.  The  toes  have  adhesive  disks.  Members  of  the  genus 


all  by  %;  4,  adult  male  xl.    (From 
Gadow,  after  Peters.) 


AMPHIBIA  203 

Dendrobatinus  carry  their  tadpoles  on  the  back  while  going  from  a  pool 
that  is  drying  up  to  one  with  plenty  of  water. 

Sub-Family  3.  Ranince,  comprises  the  typical  frogs,  to  which  our 
common  bull-frog,  leopard  frog  (Fig.  115,  E),  grass-frog,  etc.,  belong. 
These  common  frogs  are  very  well  known  to  every  student  of  zoology 
and  will  not  be  dealt  with  here.  The  RaninaB  are  remarkable  for  the 
range  of  their  adaptive  radiation.  They  range  from  purely  aquatic 
frogs  like  Rana  catesbiana  (Fig.  115,  F),  the  "bull-frog,"  to  terrestrial 
frogs  like  R.  temporaria,  the  European  brown  frog;  from  those  living 
on  the  ground  in  woods  to  those  living  in  trees,  and  even  to  fairly  good 
flying  frogs.  These  so-called  "flying  frogs"  have  fully  webbed  large 
feet  (Fig.  115,  D)  with  which  they  parachute  to  the  ground  or  from 
tree  to  tree.  There  is,  however,  considerable  doubt  as  to  their  "fly- 
ing" ability.  Their  air  leaps  cannot  be  very  great,  possibly  from  20 
to  30  feet  being  the  maximum.  Wallace's  exaggerated  account  of 
these  activities  has  been  too  widely  accepted. 

The  frog's  usual  method  of  capturing  insect  prey  is  shown  in  Fig. 
115,  G.  Any  statement  as  to  special  breeding  habits  would  be  largely 
a  repetition  of  what  has  been  said  about  other  groups. 

THE  DEVELOPMENT  OF  THE  FROG 

The  early  embryology  of  the  Amphibia  is  in  general  the  most  gen- 
eralized found  among  the  vertebrate  classes,  and  that  of  our  com- 
monest frogs  is  as  primitive  as  can  be  found.  Why  the  development  of 
the  Amphibia  is  more  primitive  than  that  of  the  fishes  is  not  an  easy 
question  to  answer.  It  appears  probable,  however,  that  the  earliest 
fishes,  such  as  the  lobe-fin  ganoids,  had  a  type  of  egg  and  a  process  of 
development  even  more  like  that  of  the  Amphibia  than  have  the  mod- 
ern fishes,  and  that  the  amphibian  descendants  of  these  ancestral 
fishes  have  retained  more  nearly  than  the  fish  descendants  the  primi- 
tive features  of  development.  A  study  of  comparative  embryology 
of  chordates  usually  begins  with  the  development  of  Amphioxus  and 
then  proceeds  directly  to  that  of  the  frog.  Then  follows  the  develop- 
ment of  the  chick,  as  an  example  of  conditions  in  the  Sauropsida,  and 
finally  that  of  a  eutherian  mammal. 

The  life  history  of  the  frog  may  conveniently  be  divided  into 
four  periods: — 

1.  The  period  of  germ-cell  formation,  which  terminates  with 
spawning. 


204 


VERTEBRATE  ZOOLOGY 


2.  The  period  of  embryonic  development,  which  begins  with  fer- 

tilization and  ends  with  hatching. 

3.  The  larval  period,  which  extends  from  hatching  to  the  comple- 

tion of  the  process  of  metamorphosis. 

4.  The  period  of  adolescence,  extending  from  the  end  of  metamor- 

phosis to  sexual  maturity. 
1.  The  period  of  germ  cell  formation  involves  both  ovogenesis 

and  spermatogenesis,   together  with  the  processes  of  maturation. 

These  stages  are  quite  regular  and  require  no  special  comment.    The 

eggs  are  laid  in  a  string, 
attached  to  one  another 
by  means  of  a  continuous 
gelatinous  envelope,  which 
is  at  first  dense  and  vis- 
cous, but  soon  absorbs 
sufficient  water  to  cause 
it  to  swell  to  several  times 
its  original  thickness.  This 
jelly,  which  is  laid  down 
in  two  layers,  has  the 
double  function  of  con- 
serving heat  for  incuba- 
tion purposes  and  of  pre- 


venting    the 
FIG.  118. — Frog's  egg  before  and  after  fertil-   bacteria, 
ization,  showing  symmetry  relations.    A,  unfer- 
tilized egg,  from  side;  B,  Unfertilized  egg,  from 
vegetal  pole;  C,  Fertilized  egg  just  before  cleav- 
age 


attacks    of 


The  fertilized  egg  (Fig. 
118)     is     rather     highly 


from  side;   D,   same   from   vegetal  pole,   organized  before  cleavage 
u,    gray  crescent;   p.  pigmented   animal   pole;   u     .       ,.      ., 
w,  unpigmented  vegetal  pole.     (From  Kellicott.)    beSms>  for  the  various  axes 


of  the  future  embryo 
(antero-posterior  axis,  dorso-ventral  axis  and  the  axis  of  bilaterality) 
are  already  clearly  defined.  These  relations  can  be  made  out  readily 
from  the  pigment  pattern  of  the  peripheral  parts  of  the  egg.  The  upper 
hemisphere  of  the  egg  is  covered  with  black  pigment,  which  is  like  an 
obliquely  placed  cap.  A  gray  crescent,  thick  at  one  side  and  fading 
out  on  the  other,  separates  the  pigmented  area  from  the  pale  yellow 
area  at  the  vegetal  pole.  Only  one  line  can  be  drawn  around  the  egg 
so  as  to  divide  it  into  bilaterally  equal  halves;  this  represents  the 
primary  axis  of  the  embryo.  The  yolk  is  abundant,  and  only  a  small 


AMPHIBIA  205 

region  at  the  apical  pole  is  free  from  yolk  granules.  Maturation  of 
the  egg  occurs  partly  before  laying,  one  polar  body  being  given  off 
during  the  descent  of  the  egg  in  the  oviduct.  The  second  maturation 
division  occurs  after  insemination. 

2.  The  Embryonic  Period  (Fig.  119). — Fertilization  occurs  while 
the  eggs  are  being  laid,  the  spear-shaped  spermatozoon  penetrating 
the  gelatinous  envelope  and  forcing  a  path  through  the  yolk  to  the  egg 
nucleus.  Cleavage  is  total  or  holoblastic  in  spite  of  the  large  accumu- 
lation of  yolk.  The  first  and  second  cleavage  furrows  being  meridional 
and  the  third  unequally  equatorial,  cutting  off  four  micromeres  from 
the  apical  and  four  macromeres  from  the  vegetal  pole  of  the  egg.  The 
micromeres  cleave  much  more  rapidly  than  the  yolk-laden  macro- 
meres,  resulting  in  the  formation  of  a  rather  thick-walled  but  fairly 
typical  llastula,  with  numerous  small  pigmented  cells  above  and  com- 
paratively few  large  unpigmented  cells  below.  The  hollow  of  the 
blastula,  or  segmentation  cavity,  is  much  reduced  in  volume  because 
of  the  thickness  of  the  cells  at  the  vegetal  pole.  Gastrulation,  while 
not  so  simple  as  in  Amphioxus,  is  clearly  homologous  with  the  latter. 
The  departure  from  the  diagrammatic  condition  is  due  to  the  accumu- 
lation of  yolk,  which  prevents  the  typical  embolic  invagination  of  the 
very  thick  layer  of  vegetal  pole  cells.  The  difficulty  is  evaded  by 
having  the  invagination  take  place  at  the  edge  of  the  thickened  area, 
where  a  flat  infolding  of  surface  cells  takes  place  just  below  the  edge 
of  the  pigmented  area,  leaving  a  crescentic  blastopore  on  the  surface. 
The  main  part  of  the  gastrulation  process  is  accomplished  by  the 
overgrowth  of  the  endoderm  cells  by  the  ectodermal  cap,  a  process 
known  as  epibolic  gastrulation.  The  archenteron.  at  first  flat  and  with- 
out a  lumen,  soon  expands  and  largely  displaces  the  segmentation 
cavity.  The  gastrula  is  morphologically  a  two-layered  embryo,  with 
a  layer  of  ectoderm  on  the  outside  and  a  layer  of  endoderm  within, 
though  in  the  frog  each  of  these  layers  is  more  than  a  single  cell  layer 
in  thickness.  Mesoderm  formation  is  accomplished  by  the  ingrowth  of  • 
a  sheet  of  cells  around  the  blastopore.  This  zone-like  layer  soon 
splits  into  two  layers,  an  outer  somatic  and  an  inner  splanchnic  layer, 
with  the  primitive  ccelomic  cavity  between.  Ccelomic  pouches  not 
unlike  those  in  Amphioxus  arise  from  the  dorsal  lateral  regions  of  the 
archenteron,  and  a  median  dorsal' strip  of  the  latter  is  left  over  to 
form  the  notochord.  The  development  of  the  central  nervous  system 
is  decidedly  precocious,  for  even  in  a  late  gastrula  stage  the  medul- 


FIG.  119. — Development  of  the  Frog.  A-F,  cleavage;  G,  overgrowth  of 
ectoderm;  H,  I,  establishment  of  germ-layers;  J,  K,  assumption  of  tadpole-form 
and  establishment  of  nervous  system,  notochord  and  enteric  canal  (archenteron)  ; 
L,  newly-hatched  tadpole,  bl.  coel,  blastoccole;  blp.  blp',  blastopore;  brr.  br", 
cutaneous  gills;  br.  cl,  branchial  arches;  e,  eye;  ect,  ectoderm;  end,  endoderm; 
erit,  enteron;/.  br,  fore-brain;  h.  br,  hind-brain;  w.  br,  mid-brain;  md.f,  medullary 
fold;  md.  gr,  medullary  groove;  mes,  mesoderm;  mg,  megameres;  mi,  micromeres; 
nch,  notochord;  n.  e.  c,  neurenteric  canal;  pcdm,  proctoda3um;  pty,  pituitary  in- 
vagination;  ret,  commencement  of  rectum;  sk,  sucker;  sp.  cd,  spinal  cord;  stdm, 
stomodaeum;  t,  tail;  yk,  yolk  cells;  yk.  pi,  yolk  plug.  (From  Parker  and  Haswell, 
after  Ziegler  and  Marshall.) 

206 


AMPHIBIA  207 

lary  plaie  is  clearly  defined.  At  a  time  when  the  blastopore  is  nearly 
closed  the  dorsal  parts  of  the  embryo  show  the  broad  primitive  groove, 
flanked  on  both  sides  by  two  pairs  of  medullary  folds,  inner  and  outer. 
The  outer  folds  fade  away,  but  the  inner  ones  arch  over  the  groove 
and  meet  in  the  region  of  the  future  neck,  the  closure  proceeding 
thence  backwards.  Thus  the  groove  is  converted  into  the  neural  tube. 
The  anterior  part  of  the  tube  soon  becomes  differentiated  into  the 
primitive  brain,  with  the  three  primary  brain  lobes  representing  the 
primordia  of  the  fore,  mid,  and  hind  brain.  During  these  changes  the 
embryo  has  been  elongating  and  before  hatching  has  reached  a  length 
nearly  three  times  its  breadth. 

3.  Larval  Period  (Fig.  120,  1-10).— At  the  time  of  hatching  the 
larva  is  a  somewhat  fish-like  creature  with  a  fairly  long  vertically 
flattened  tail.  The  mouth  is  ventral  in  position  and  is  soon  surrounded 
by  a  chitinous  rim  or  scraper,  which  is  used  as  a  larval  organ  in  scrap- 
ing off  nutritive  scum  from  lily  pads,  etc.  Two  pairs  of  branching 
external  (larval)  gills  are  the  first  functional  respiratory  organs.  In 
addition  to  the  external  gills,  internal  gills,  homologous  with  those  of 
adult  fishes,  are  formed  and  take  over  the  respiratory  function  for  a 
considerable  period.  Soon  the  external  gills  disappear,  and  a  fold  of 
skin  grows  backward  from  in  front  of  their  original  location,  forming 
an  operculum  under  which  lie  the  internal  gills.  The  operculum  has 
but  one  outlet,  a  small  unpaired  spiracle  on  the  left  side.  Some 
writers  have  interpreted  this  operculum  as  the  equivalent  of  the 
atrium  of  Amphioxus,  but  the  homology  has  not  been  fully  estab- 
lished. The  hind  limbs  are  the  first  to  appear,  closely  followed 
by  the  fore  limbs,  which  for  some  time  are  concealed  beneath 
the  operculum.  Only  in  the  later  stages  of  larval  life  are  the  lungs 
developed,  and  as  long  as  the  larva  uses  the  gills  the  lungs  remain 
very  small. 

Metamorphosis  (Fig.  120,  11-15). — The  period  of  metamorphosis 
is  really  a  part  of  the  larval  period  and  cannot  be  sharply  marked  off 
from  the  latter,  since  the  change  is  a  gradual  one.  Toward  the  close 
of  the  larval  period  the  tail  begins  to  be  resorbed  and  its  materials 
are  stored  up  in  the  liver.  The  long,  spirally  coiled  intestine  shortens. 
The  mouth  loses  its  chitinous  rim  and  grows  much  wider.  The  gills 
disappear  and  the  lungs  grow  rapidly  in  size  and  the  larva  comes 
frequently  to  the  surface  to  breathe  air.  When  these  changes  are 
complete  the  animal  is  no  longer  a  larva,  but  an  adolescent  frog. 


FIG.  120. — Metamorphosis  of  the  Frog.  1,  tadpole  just  hatched,  dorsal  aspect; 
2-3,  older  tadpoles,  side  view;  4,  later  tadpole,  dorsal  view  showing  cutaneous 
gills  at  maximum  development;  5,  later  stage,  ventral  view,  showing  degeneration 
of  cutaneous  gills  and  development  of  operculum;  6,  older  tadpole,  left  side, 
showing  single  opening  of  operculum;  7,  older  stage,  right  side,  showing  hind  leg 
and  anus;  8  and  10,  lateral  views  of  two  later  stages  showing  development  of  hind 
legs;  9,  dissection  of  tadpole  to  show  internal  gills,  spiral  intestine,  and  anterior 
legs  developed  within  operculum;  11,  advanced  tadpole  just  before  metamorpho- 
sis; 12,  13,  14,  stages  in  metamorphosis,  showing  gradual  resorption  of  tail;  15, 
juvenile  frog  after  metamorphosis.  (From  Weysse,  after  Leuckart-Nitsche  wall 
chart.) 

208 


AMPHIBIA  209 

Some  species  of  frog  go  through  to  metamorphosis  in  a  few  weeks, 
others  require  months,  and  some  require  two  or  three  years. 

4.  The  Period  of  Adolescence  has  not  been  very  fully  studied. 
It  is  a  long,  slow  process  involving  changes  in  the  relative  proportions 
of  the  parts,  elaborations  of  the  histological  structure,  and  ossification 
of  the  cartilaginous  skeleton.  The  most  significant  changes  are  those 
that  are  last  to  take  place,  namely,  those  that  have  to  do  with  the 
onset  of  sexual  maturity.  Shortly  before  the  beginning  of  the  first 
breeding  season,  the  cells  of  the  ovaries  and  testes  begin  the  processes 
known  as  ovogenesis  and  spermatogenesis,  that  constitute  the  chief 
features  of  the  first  period  considered  in  this  brief  life  history.  Hence 
we  have  completed  the  cycle  for  one  generation. 


CHAPTER  VII 
CLASS  IV.     REPTILIA 

It  is  generally  conceded  that  of  all  the  vertebrate  classes  the  Rep- 
tilia,  past  and  present,  stand  foremost  in  numbers,  in  size,  in  range 
of  specialization  and  in  dominance  in  the  organic  world.  Although 
the  reptiles  of  the  present  (crocodiles,  turtles,  lizards  and  snakes) 
play  a  comparatively  unimportant  role  in  the  realm  of  nature,  those 
of  the  past  were  frequently  of  giant  proportions  and  were  the  dreaded 
tyrants  of  the  earth,  of  the  waters,  and  to  some  extent  of  the  air.  The 
golden  age  of  the  reptiles  was  the  Mesozoic,  which  has  therefore  been 
called  the  "  Age  of  Reptiles. "  After  a  modest  career  during  the  early 
period  of  the  Mesozoic,  several  specialized  groups  arose  and  under- 
went a  remarkable  adaptive  radiation  into  all  of  the  principal  life 
zones.  Perhaps  the  most  remarkable  of  these  specialized  assemblages 
was  that  of  the  dinosaurs,  a  group  that  for  millions  of  years  held  sway 
over  the  earth  to  an  extent  equaled  only  by  that  of  Man  to-day.  Dur- 
ing this  period  other  orders  of  reptiles  played  only  a  secondary  role. 

Dramatic  as  was  this  rise  to  dominance  of  the  greater  reptilian 
orders  during  the  Mesozoic,  their  sudden  extinction  at  or  near  the 
close  of  this  age  was  even  more  remarkable.  After  an  unprecedented 
reign  as  autocrats  of  earth  and  sky  and  sea  for  a  period  of  not  less  than 
ten  millions  of  years,  they  abruptly  ceased  to  be.  The  causes  of  their 
extinction  are  unknown  and  we  can  only  vaguely  conjecture  that  they 
died  off  for  no  better  reason  than  that  they  had  run  their  course,  had 
reached  the  limits  of  their  various  lines  of  specialization,  had  become 
stereotyped,  senescent,  and  could  evolve  no  further.  To  use  an  idea 
of  Osborn's,  they  had  proceeded  to  the  end  of  an  evolutionary  cul-de- 
sac  from  which  there  was  no  egress. 

Only  the  crocodiles,  turtles,  lizards  and  snakes,  among  reptiles, 
were  sufficiently  generalized  to  weather  the  crisis  and  live  on  into  the 
Cenozoic  age  to  be  the  contemporaries  of  the  birds  and  the  mammals, 
which  are  the  dominant  orders  of  that  period.  These  modern  groups 
have  evidently  carried  on  a  war  of  destruction  against  the  reptiles, 
and  still  the  unequal  struggle  goes  on,  with  the  reptiles  on  the  losing 

210 


REPTILIA 


211 


side.  The  reptiles  are  doomed.  Between  the  birds  of  the  air  and  the 
beasts  of  the  field  and  forest,  and  especially  at  the  hand  of  that  super- 
mammal,  Man,  who  seems  to  have  centered  his  aversion  upon  the  ser- 
pents, the  reptiles  are  destined  to  oblivion,  except  in  so  far  as  Man 
sees  fit  to  preserve  certain  types  for  his  own  uses.  No  mere  verbal 
/  V  // 


FIG.  121. — Chart,  showing  origin  and  adaptive  radiation  of  the  reptiles.  Dotted 
areas  represent  existing  groups,  black  areas,  extinct  groups.  This  chart  also 
shows  the  origin  of  the  birds  and  mammals  from  reptilian  stock.  In  the  cases  of 
several  modern  groups  (Chelonia,  egg-laying  mammals,  placental  mammals, 
Sphenodon  and  Crocodiles)  the  dotted  areas  should  reach  the  top.  (After  Os- 
born's  "Origin  and  Evolution  of  Life.") 

description  shows  so  vividly  the  origin  and  adaptive  radiation  of  rep- 
tiles as  does  the  accompanying  chart  (Fig.  121). 

From  the  dawn  of  the  reptiles  during  the  Palaeozoic  up  to  the  pres- 
ent there  have  passed  between  fifteen  and  twenty  millions  of  years, 
an  immense  period  as  compared  with  the  brief  span  of  Man's  life  upon 
the  globe.  The  history  of  the  rise  and  fall  of  the  reptilian  empire  is 
one  of  giant  proportions  and  of  intense  dramatic  interest.  Only  the 
vertebrate  palaeontologist  is  in  a  position  adequately  to  picture  this 
drama  for  us. 

EVOLUTIONARY  ADVANCES  MADE  BY  THE  REPTILES 

"The  environment  of  the  ancestor  of  all  the  reptiles,"  says  Os- 
born,  "  was  a  warm,  terrestrial,  and  semirandg-egion,  favorable  to  a 
sensitiy£jier^ou^sy;stem;  alert  motions^scaly  amiature,  slender  limbs, 


212  VERTEBRATE  ZOOLOGY 

a  vibratile  tail,  and  the  capture  of  food  both  by  sharply  pointed,  re- 
curved~tee£h  and  by  the  claws  of  a  five-fingered  hand  and  foot." 

The  essential  evolutionary  advance  which  the  reptiles  made  upon 
the  Amphibia  had  to  do  with  the  total  abandonment,  from  the  very 
beginning  of  development,  of  the  aquatic  habit  of  life.  The  Am- 
phibia were,  and  still  are,  for  the  most  part,  dependent  upon  water  dur- 
ing at  least  a  considerable  portion  of  their  life  cycle;  the  Reptilia  were 
from  the  first  quite  independent  of  an  aquatic  environment.  This 
emancipation  from  the  need  of  an  aquatic  habitat  was  accomplished  in 
three  ways :  by  the  acquisition  of  lungs  of  a  more  adequate  sor^  by  a 
scaly  covering  to  prevent  dessication;  by  the  development  of  impor- 
tant embryonic  membranes,  the  amnion  and  the  allantois,  which 
are  merely  adaptations  for  embryonic  development,  in  the  air/ 

Undoubtedly  the  Amphibia  had  made  considerable  progress  in  the 
direction  of  adaptations  for  adult  life  on  land,  for  they  often  have  as 
good  lungs  as  do  some  of  the  reptiles.  Moreover,  many  of  the  ex- 
tinct Amphibia  had  an  adequate  scaly  covering.  There  is  no  evi- 
dence, however,  that  any  of  the  Amphibia  have  ever  acquired  the 
capacity  of  reproducing  on  the  dry  land,  except  in  connection  with 
some  peculiar  brooding  adaptation,  such  as  that  seen  in  some  of  the 
tree-toads.  The  fact  that  all  modern  Amphibia  have  functional  gills 
at  some  period  adds  force  to  this  last  statement. 

The  reptiles  have  entirely  given  up  gill  respiration,  as  is  evidenced 
by  the  total  absence,  except  for  minute  transitory  traces,  of  gill 
tissue  at  any  stage  of  development.  In  place  of  gills  the  embryo  uses 
the  allantois,  an  extensive  embryonic  lung.  To  avoid  dessication  and 
the  possibility  of  contact  injuries,  the  embryo  is  surrounded  by  a 
fluid-filled  sac,  made  from  folds  of  its  own  tissues.  This  veritable 
private  aquarium  is  called  the  amnion,  a  structure  adopted  by  all 
land  vertebrates  for  development  in  the  air. 

The  amnion  and  the  allantois  then  are  structures  of  first  rate 
importance  in  connection  with  the  origin  and  evolution  of  land  verte- 
brates, and  especially  of  the  Reptilia.  The  egg  of  the  reptile  is  a 
large  object,  much  like  that  of  the  bird.  It  is  provided  with  a  tough 
shell  for  protection  and  a  thick  layer  of  albumen  for  nutriment.  The 
egg-yolk  is  abundant  and  serves  as  the  main  food  supply  of  the  grow- 
ing embryo.  There  is  no  real  larval  stage,  for.  the  newly  hatched 
young  is  essentially  like  the  adult  except  in  the  relative  proportions 
of  head  and  body  and  in  being  sexually  immature. 


- 


REPTILIA 


213 


The  amnion  (Fig.  122), 
which  is  formed  very  early, 
results  from  the  fusion  to- 
gether of  two  lateralrfolds  of 
the  extra-embryonic  blas- 
toderm and  is  a/  sort  of 
bladder-like  membrane  con- 
taining a  watery/  fluid  in 
which  the  growing  embryo 
lies.  The  fluid  within  the 
amnion  increases  greatly  in 
amount  and  provides  an 
ample  space  for  the  further 
growth  of  the  embryo,  and 
preserves  the  latter  from  con- 
tacts, mechanical  abrasions, 
or  other  mechanical  injuries. 
The  allantois  (Fig.  122) 
starts  as  a  finger-like  diver- 
ticulum  of  the  embryonic 
hind-gut  and  grows  out  ex- 
tensively  between  the  amnion 
and  the  chorion  as  a  large 
umbrella-shaped  sac,  which 

is   highly    vascular;    and    it       FlQ    122._Diagrams   of   the    embryonic 
functions    as    an    embryonic    membranes,  amnion,   allantois,   yolk-sac,  of 
lung.     The  amnion   and    the   Amniotes.   A,  Sauropsida  (reptiles  and  birds). 
TI      ,    •  ,v  i        i       B,    mammal    with    primitive    allantoic   pla- 

allantois  together  make  de-  ce'nta     (From  Lull>  J^  Wilder } 

velopment   possible  even   in 

arid  regions  and  give  to  the  reptiles  a  decided  advantage  in  avail- 
able range,  since  they  may  live  far  from  the  water. 

The  Earliest  Reptiles  and  Their  Origin 

"Just  when  the  animals  we  call  reptiles  arose  in  geological  history 
we  do  not  know;  certainly  it  was  in  early  Pennsylvanian  (Upper  Car- 
boniferous) times,  probably  Mississippian  (Lower  Carboniferous). 
That  they  arose  from  what  we  call  the  Amphibia,  forms  with  temno- 
spondylous  vertebrae,  is  certain,  though  there  is  not  much  more  reason 
for  calling  the  ancestral  stock  Amphibia  than  Reptilia.  I  prefer  to 


214  VERTEBRATE  ZOOLOGY 

call  it  provisionally,  Protopoda.  It  was  ancestral  to  both  and  both 
classes  have  advanced  since  their  divergence,  the  Amphibia  some,  the 
Reptilia  much.  Could  we  find,  as  some  time  we  hope  that  we  may, 
in  mid-Mississippian  or  late  Devonian  times,  a  skeleton  of  those 
ancestral  creatures,  we  should  perhaps  not  call  it  by  the  name  of  any 
known  order;  it  would  be  the  old  question  over  again  of  the  difference 
between  animals  and  plants.  At  present  we  know  the  Protopoda 
only  by  their  footprints." 

This  statement  of  Williston  is  somewhat  radical  in  that  it  places 
the  Amphibia  and  the  Reptilia  on  the  same  level,  neither  being  ances- 
tral to  the  other,  but  both  derived  from  a  common  ancestor  about 
which  we  know  nothing  except  the  characters  revealed  by  its  foot- 
prints. Some  authors  have  assumed  that  this  creature  was  an  ances- 
tral amphibian,  a  view  that  has  gained  wide  acceptance.  For  our  pur- 
poses it  seems  advisable  to  think  of  the  creature  that  made  the 
footprints  (Fig.  101)  as  the  earliest  known  land  vertebrate,  and  to 
call  it  the  ancestral  amphibian. 

Palaeontologists  generally  agree  that  the  reptiles  go  back  nearly 
as  far  as  do  Amphibia,  and  that  their  evolution  has  been  to  a  large 
extent  parallel.  Both  experimented  with  adaptations  for  land  life 
and  the  reptiles  were  much  more  successful  in  these  ventures  than 
were  their  rivals.  During  the  Permian,  however,  they  were  neck-and- 
neck  in  the  race;  for  every  reptilian  type  of  that  period  there  was  a 
parallel  amphibian  type.  The  reptiles  finally  outstripped  the  Am- 
phibia, largely  through  their  adoption  of  the  amnion-allantois  com- 
plex and  their  consequent  emancipation  from  the  water. 

PERMIAN  REPTILES 

The  reptiles  of  the  Permian  were  partly  archetypal  forms  and  partly 
precociously  specialized  and  already  senescent  types.  Williston  finds 
four  assemblages  of  Palaeozoic  reptiles  (belonging  to  the  American 
Permo-Carboniferous).  Each  of  these  assemblages  has  evidently  the 
systematic  value  of  a  sub-class  and  has  one  or  more  modern  repre- 
sentatives. 

a.  Diapsida;  represented  to-day  by  Sphenodon,  crocodiles,  birds 
•        an^fahe  great  extinct  dinosaurs,  pterosaurs,  parasuchians,  etc. 

b.  Synapsida ;  represented  to-day  by  the  mammals,  and  by  the 

extinct   plesiosaurs,    anomodonts,    therapsidans   and   ther- 
omorphs,  etc 


REPTILIA 


215 


c.  Parapsida;  represented  to-day  by  the  lizards  and  snakes,  and 

by  the  extinct  mososaurs,  ichthyosaurs,  etc. 

d.  Anapsida;  represented  to-day  by  the  turtles,  and  by  the  extinct, 

cotylosaurs  and  the  extinct  semi-chelonian,  Eunotosaurus. 
Thus  we  see  that  the  reptiles  had  undergone  an  extensive  adaptive 
radiation  even  in  the  Palaeozoic.      Of  the  several  adaptive  types  of 


FIG.  123. — Group  of  Palaeozoic  Reptilia.  A,  Varanops;  B,  Labidosaurus; 
C,  Seymouria;  D,  Dimetrodon;  E,  Cynognathus  (a  mammal-like  reptile) ;  F,  head 
of  Scymnognathus  (a  South- African  "dog-toothed"  reptile).  (Redrawn  from 
Osborn,  after  Williston  and  after  Gregory.) 

that  period  perhaps  the  most  significant  is  that  exemplified  by  the 
lizard-like  reptile  Varanops  (Fig.  123,  A),  a  creature  so  generalized 
that  it  might  well  be  selected  as  an  archetypal  reptile.  The  fact  that 
it  has  a  skull  and  skeleton  much  like  that  of  modern  lizards  has  led 
Williston  to  the  conclusion  that  some  of  our  modern  lizards  are  more 


216  VERTEBRATE  ZOOLOGY 

primitive  than  the  classic  Sphenodon,  long  thought  of  as  the  most 
pj  imitive  of  living  reptiles.  Varanops  and  its  more  slender  relatives 
represent  the  quick-running  or  cursorial  adaptation  as  it  appeared 
within  the  sub-class  Parapsida. 

As  examples  of  the  secondarily  semi-aquatic  adaptive  types  we  may 
cite  several  members  of  the  sub-class  Anaspida,  such  as  Labidosaurus 
(Fig.  123,  B),  Seymouria,  (Fig.  123,  C),  and  Diadectes,  three  types  that 
probably  lived  much  as  do  our  modern  frogs  and  salamanders.  As  an 
example  of  the  heavy-bodied,  and  heavily  armored  type  we  may  cite 
two  members  of  the  sub-class  Synapsida:  Edaphosaurus  and  Dimet- 
trodon  (Fig.  123,  D),  reptiles  strikingly  characterized  by  a  riotous 
growth  of  dorsal  spines.  These  so-called  pelycosaurs  evidently  rep- 
resent an  end  product  of  a  very  early  line  of  specialization  and  have 
left  no  descendants. 

The  Mammal-Like  Reptiles  (Cynodonts). — Another  remarkable 
group  of  Permian  reptiles  which  appears  to  have  been  purely  African 
in  distribution  was  a  group  of  mammal-like  reptiles,  called  Cynodon- 
tia,  believed  by  the  authorities  to  have  given  rise  to  the  line  from  which 
the  mammals  arose.  These  cynodonts  (dog-toothed  reptiles)  showed 
many  tendencies  toward  mammalian  conditions,  chief  among  which 
were :  heterodont  dentition  (a  specialization  of  the  teeth  into  incisors, 
canines  and  molars),  more  effective  types  of  limbs  for  rapid  land  loco- 
motion, a  tendency  for  the  angulare  and  articulare  bones  of  the  lower 
jaw  to  disappear,  and  a  tendency  for  the  skull  to  become  completely, 
roofed  over  and  for  the  so-called  vacuities  to  disappear.  These  cyno- 
donts were  evidently  carnivorous  types  of  which  Cynognathus  (Fig. 
123,  E)  is  a  good  example.  The  head  of  another  cynodont,  Scymnog- 
nathus  (Fig.  123,  F)  shows  clearly  the  dog-like  dentition.  These 
reptiles  are  once  more  to  claim  our  attention  when  we  come  to  discuss 
the  question  of  the  origin  of  mammals. 

There  is  reason  to  believe  that  at  least  five  or  six  other  reptilian 
orders  had  representatives  in  the  Permian  or  Permo-Carboniferous: 
Chelonia  (turtles),  plesiosaurs  (aquatic  reptiles),  ichthyosaurs  (fish- 
like  reptiles),  Squamata  (primitive  lizards),  Rhynchocephalia  (beaked 
reptiles),  and  Parasuchia  (primitive  crocodiles).  Possibly  also  the 
great  order  of  dinosaurs  had  its  beginnings  in  the  Permo-Carbonifer- 
ous, though  as  yet  there  is  no  direct  evidence  of  their  presence  during 
this  period. 

A  number  of  orders  of  reptiles  not  only  had  their  origin  during  the 


REPTILIA  217 

Palaeozoic,  but  actually  ran  out  their  entire  course  of  specialization 
and  became  entirely  extinct  before  the  Mesozoic  age  began.  Thus  the 
cotylosaurs,  proganosaurs,  anomodonts,  pelycosaurs,  and  phytosaurs 
died  out  either  in  Permian  or  at  least  not  later  than  early  Triassic 
times.  The  remaining  orders  that  arose  in  the  Palaeozoic  were  able 
to  weather  the  climatic  crisis  at  the  end  of  this  age  and  were  the  an- 
cestors of  the  great  Mesozoic  orders  of  reptiles. 

THE  GOLDEN  AGE  OF  REPTILES 

The  reptiles,  as  we  have  seen,  made  a  modest  start  in  the  Carbonifer- 
ous, underwent  a  considerable  degree  of  adaptive  specialization  during 
the  Permian,  and  in  some  lines  became  senescent  and  died  out.  On 
the  whole,  however,  the  Palaeozoic  reptiles  were  of  generalized  or 
primitive  types  and  gained  no  great  ascendency.  It  was  not  until 
the  Mesozoic  that  the  reptiles  really  came  into  their  own.  It  was 
during  this  "Age  of  Reptiles,"  an  immense  period  involving  several 
millions  of  years,  that  they  gained  their  world  supremacy  and  came 
to  exercise  undisputed  sway  over  the  land  habitats,  and  disputed 
with  the  fishes  the  right  to  rule  the  waters.  The  dominance  of  the 
reptiles  of  this  period  was  due  largely  to  five  great  groups:  ichthyo- 
saurs,  plesiosaurs,  carnivorous  and  herbivorous  dinosaurs,  and  ptero- 
saurs. Each  of  these  assemblages  deserves  individual  attention. 

ICHTHYOSAURIA 

No  more  extreme  case  of  adaptation  of  a  member  of  an  essen- 
tially terrestrial  class  for  an  aquatic  habitat  could  be  given.  The 
first  reptiles  are  believed  to  have  acquired  their  main  characters  in 
adaptation  to  land  life,  so  we  have  no  alternative  than  to  believe 
that  the  ichthyosaurs  have  been  derived  from  land  forms  that  found 
the  sea  a  rich  hunting  ground  and  developed  the  habiliments  of  a  fish 
to  facilitate  their  aquatic  activities.  A  change  involving,  first,  adapta- 
tions for  land  life  and,  second,  a  return  of  aquatic  adaptations  is  re- 
ferred to  as  an  example  of  reversed  aquatic  adaptation,  and  is  by  no 
means  uncommon  among  the  higher  vertebrates.  The  external  form 
of  an  ichthyosaur  (Fig.  124,  C)  is  strikingly  like  that  of  a  sword-fish. 
The  pectoral  and  pelvic  limbs  are  flipper-like  fins,  the  tail  has  a  re- 
markable caudal  fin  externally  precisely  like  that  of  a  fish,  a  very 
fish-like  dorsal  fin  plays  the  same  role  as  that  in  a  fish.  Unlike  other 
reptiles  and  like  the  fishes,  these  creatures  have  no  real  neck,  but  the 
head  seems  to  be  joined  broadly  with  the  trunk.  All  of  these  adapta- 


218 


VERTEBRATE  ZOOLOGY 


tions  are  utterly  fish-like,  but  the  fundamental  internal  anatomy  of 
the  creature  is  essentially  reptilian.  It  seems  probable  that  the  ich- 
thyosaurs  were  viviparous,  as  are  some  purely  aquatic  reptiles  of 


FIG.  124. — Group  of  Mesozoic  Reptilia.  A,  Long-necked  plesiosaur,  Elas- 
mosaurus;  B,  short-necked  piesiosaur,  Trinaciomerion;  C,  ichthyosaur,  Bap- 
lanodon;  D,  pterodactyl;  E,  "Ostrich"  dinosaur,  Struthiomimvs;  F,  carnivorous 
dinosaur,  Tyrannosavrus;  G,  giant  herbivorous  dinosaur,  Brachiosaurus;  fl, 
hooded  "duck-bill"  dinosaur,  Coryihosaurus.  (Redrawn  after  Osborn.) 

to-day;  for  it  is  inconceivable  that  creatures  so  purely  aquatic  in 
habits  should  come  ashore  to  lay  their  eggs. 


REPTILIA  219 


PLESIOSAURIA 

These  marine  reptiles  furnish  less  extreme  examples  of  aquatic 
adaptations  than  do  the  ichthyosaurs.  Some  of  the  plesiosaurs 
reached  a  giant  size,  being  upwards  of  fifty  feet  in  length.  The  early 
members  of  this  group  were  of  a  rather  generalized  type  and  might 
properly  have  been  thought  of  as  marine  lizards.  Later  came  a  type 
such  as  Elasmosaurus  (Fig.  124,  A),  a  slow-moving,  long-necked, 
short-bodied,  small-headed  type,  with  long,  narrow  paddles.  The  cli- 
max of  plesiosaurian  specialization  was  reached  by  such  forms  as  Tri- 
nacromerion  (Fig.  124,  B),  which  is  characterized  by  short  body,  rather 
short  neck,  and  fiercely  predaceous  jaws;  a  creature  with  all  the  ear- 
marks of  an  aquatic  speed  demon,  and  doubtless  as  much  of  a  terror  to 
the  fishes  as  were  the  dinosaurs  to  the  smaller  denizens  of  the  dry  land. 

CARNIVOROUS  DINOSAURS 

The  dinosaurs  are  the  last  word  in  terrestrial  specialization  among 
the  reptiles.  Doubtless  the  ancestors  of  this  great  group  were  rather 
generalized  lizard-like  forms  that  lived  in  the  late  Permian,  but  as  yet 
the  paleontologists  have  not  been  able  to  place  their  hands  upon  an 
unequivocal  ancestral  dinosaur.  As  has  already  been  said,  the  group 
had  a  dramatic  rise  to  dominance  and  an  equally  dramatic  extinction 
at  the  close  of  the  Cretaceous.  While  they  lasted,  their  course  was 
an  impressive  one  and  far  out-shadowed  that  of  all  contemporaneous 
land  creatures.  On  this  account  the  middle  and  late  Mesozoic  period 
has  with  some  justification  been  called  the  "age  of  dinosaurs." 

The  carnivorous  dinosaurs  were  for  the  most  part  Saunschia  (with 
lizard-like  pelvis),  as  opposed  to  the  Ornithiscia  (with  bird-like  pel- 
vis), a  group  to  which  most  of  the  herbivorous  dinosaurs  belong.  The 
principal  evolutionary  changes  that  took  place  within  the  group  of 
carnivorous  dinosaurs  are  associated  with  an  absolute  increase  in 
body  size,  relative  decrease  in  the  size  of  the  fore  limbs  and  increase 
in  that  of  the  hind  limbs,  accompanied  by  a  progressive  tendency 
toward  bipedal  locomotion  and  speed  of  running.  The  culminating 
types  were  swift,  cursorial  creatures,  with  long  tails  for  balancing, 
short  grasping  fore  limbs,  long  neck,  and  head  armed  with  heavy 
recurved  teeth.  One  can  readily  imagine  them  as  able  to  use  their 
powerful  hind  legs  as  effectively  as  does  the  ostrich.  As  a  climax  type 
we  may  cite  the  great  Tyrannosaurus  rex  (Fig.  124,  F)  of  which  Mat- 


220  VERTEBRATE  ZOOLOGY 

thew  says:  "It  reached  a  length  of  47  feet  and  in  bulk  must  have 
equalled  the  mastodon  or  the  largest  living  elephants.  The  massive 
hind  limbs,  supporting  the  whole  weight  of  the  body,  exceeded  the 
limbs  of  the  great  proboscidians  in  bulk."  It  stood  about  20  feet 
high,  had  a  head  over  four  feet  long,  teeth  three  to  six  inches  long 
and  an  inch  wide.  The  claws  on  the  hind  feet  were  about  eight 
inches  in  length  and  of  massive  proportions.  One  can  readily  im- 
agine a  scene  of  carnage,  the  like  of  which  the  modern  animal  world 
cannot  afford,  when  such  a  reptilian  dreadnought  went  into  action 
against  one  of  those  huge,  heavily  armed,  monitor-like  reptiles  such 
as  the  herbivorous  dinosaur,  Triceratops  (Fig.  128).  Such  a  struggle 
would  decide  the  question  of  supremacy  between  offensive  andcie- 
fensive  armaments. 

THE  HERBIVOROUS  DINOSAURS  (Sauropoda). — The  contrast  be- 
tween the  carnivorous  dinosaurs  and  the  herbivorous  dinosaurs  in- 
volves largely  the  matter  of  relative  speed,  of  offensive  and  defensive 
equipment.  While  haste  is  the  essence  of  success  in  raptorial 
life,  no  time  element  is  involved  in  securing  plant  food.  It  ap- 
pears to  be  certain  that  the  early  herbivores  and  early  carnivores 
were  quite  similar  and  that  both  were  more  or  less  bipedal.  While 
the  carnivores  carried  this  cursorial  tendency  to  such  an  extreme  that 
the  fore  limbs  were  reduced  to  weak  grasping  appendages,  useless  for 
locomotion,  the  herbivores  eventually  underwent  a  reversed  evoluv 
tion  and  became  secondarily  quadrupedal.  Evident  traces  of  the  bi- 


FIG.  125. — Brontosaurus.    (From  Lull.) 

pedal  habit,  however,  are  to  be  noted  in  the  general  body  form  and  the 
relative  proportions  of  the  fore  and  hind  limbs,  the  latter  in  most  forms 
being  much  larger.  An  interesting  series  of  massive  forms  appeared 
of  which  Brontosaurus  (Fig.  125)  is  typical.  This  great  creature, 
with  a  total  length  of  nearly  a  hundred  feet,  has  comparatively  small 
fore  legs,  but  they  more  nearly  equal  the  hind  limbs  than  in  some  of 
the  earlier  members  of  this  group.  Evidently  the  fore  limbs  later 


REPTILIA  221 

underwent  a  secondary  increase  in  proportions,  for  the  culminating 
type  of  the  Sauropoda,  Brachiosaurus  (Fig.  124,  G),  had  the  fore  legs 
even  heavier  and  longer  than  the  hind  legs.  This  immense  creature 
rivaled  the  modern  whales  for  sheer  bulk,  and  was  possibly  the  most 
ponderous  creature  of  all  time;  unquestionably  it  was  the  largest 
by  all  odds  of  the  known  terrestrial  giants,  dwarfing  the  largest  ele- 
phants by  contrast. 

"In  the  final  extinction  of  the  herbivorous  sauropod  type,"  says 
Osborn,  "we  find  an  example  of  the  law  of  elimination,  attributed  to 
the  fact  that  these  types  had  reached  a  cul-de-sac  of  mechanical  evo- 
lution from  which  they  could  not  adaptively  emerge  when  they  en- 
countered in  all  parts  of  the  world  the  new  environmental  conditions 
of  advancing  Cretaceous  time." 

The  Ornithischia. — While  both  of  the  groups  of  dinosaurs  just 
described  are  alike  in  having  the  typical  reptilian  pelvis  and  are 
therefore  grouped  together  as  Saurischia,  another  great  group  of 
contemporary  dinosaurs  had  the  avian  type  of  pelvis  and  are  called 
Ornithischia.  These  dinosaurs  appear  to  have  been  an  offshoot  of  the 
early  herbivorous  types  and  had  retained  that  habit,  using  probably 
the  harder  vegetable  foods,  as  is  attested  by  the  development  of  a 
heavy,  chitinous  beak  much  like  that  of  the  modern  bird.  These 
Beaked  Dinosaurs  radiated  adaptively  into  three  distinct  structural 
types:  Ornithopoda  (bird-footed),  Stegosauria  (armored  dinosaurs), 
and  Ceratopsia  (horned  dinosaurs). 

The  Ornithopoda  were  unarmed,  bipedal  forms  doubtless  capable 
of  great  speed.  As  examples  of  these  forms  we  may  cite  the  fam- 
iliar Iguanodon  (Fig.  126)  Trachodon,  the  duck-billed  dinosaur,  and 
Corythosaurus  (Fig.  124,  H)  the  "  hooded  duck-bill. "  All  of  them  show 
well  the  bird-like  feet  and  pelvis.  It  is  significant  in  this  connection 
to  note  that  it  is  from  this  group  that  some  authors  would  derive 
the  birds,  a  theory  that  is  presented  in  discussing  the  ancestry  of 
the  bird*  Of  all  of  the  Ornithischia  perhaps  the  most  bird-like 
type  is  the  "Ostrich  Dinosaur"  Struthiomimus  (Fig.  124,  E). 

The  Stegosauria  seem  to  have  reacquired  the  quadrupedal  habit 
in  correlation  with  the  massive  weight  of  their  armature.  Stegosaurus 
(Fig.  127)  represents  the  culmination  of  the  evolution  of  the  armored 
types,  and  has  been  fittingly  chosen  as  the  exemplar  of  one  type 
of  preparedness,  distinguished  by  possessing  "all  armor  and  no  brain," 
an  equipment  as  little  apt  to  meet  with  success  in  those  antique  days 


222 


VERTEBRATE  ZOOLOGY 


as  in  the  strenuous  present.     Though  one  of  the  most  grotesque  of 
nature's  excesses  in  all  of  its  curious  make  up,  its  central  nervous 


FIG.  126.— Restoration  of  Igugftodon.    (From  Lull,  after  Heilmann.) 
PCtJa/  epthfwft'e*  d^C    FS//*J    O  b*e>X«<*l>5(ai^  *<?veh  kkvWjdoBHf? 

system  presents  one  of  the  prize  oddities  within  the  field  of  biology. 
It  is  literally  practically  brainless,  for  the  nervous  content  of  its  cra- 


FIG.  127. — Restoration  of  the  armored  dinosaur,  Stegosaurus.     (From  Lull, 
after  Schuchert.) 


REPTILIA 

nium  could  not  have  weighed  more  than  about  two  ounces,  a  brain 
that  even  a  two  weeks'  old  kitten  might  be  ashamed  of.  An  elephant 
of  comparable  size  has  a  brain-weight  of  at  least  eight  pounds.  Curi- 
ously enough  the  animal's  real  "brain,"  if  we  may  call  itjsuch,  is 
situated  near  the  base  of  its  enormous  tail,  where  there  is  a  great  sa- 
cral enlargement  of  the  spinal  chord  many  times  as  large  as  the 
dwarfed  brain.  Such  a  creature,  with  his  brain  in  his  rump,  must 
have  been  nothing  but  a  bulky,  ponderous  automaton,  driven  by 
stimuli  arising  in  the  lower  nervous  centers. 

The  Ceratopsia  were  creatures  whose  proportions  suggest  those 
of  the  rhinoceros.  Triceratops  (Fig.  128),  a  classic  example  of  the 
group,  was  about  twenty-five  feet  in  length  and  about  ten  feet  in 


FIG.  128. — Restoration  of  the  horned  dinosaur,  Triceratops.  (From  Lull,  after 
tf>chuchert.) 

height.  The  head  was  exceptionally  massive,  nearly  eight  feet  in 
length,  with  a  wide  frill-like  expansion  of  the  skull,  which  extended 
like  a  shield  over  the  neck  and  shoulders.  On  the  front  there  were 
three  great  horns,  one  on  the  snout  and  two  above  the  eyes.  Doubt- 
less such  creatures  as  these  had  as  enemies  the  great  carnivorous  di- 
nosaurs, for  no  other  contemporaneous  animals  would  have  necessi- 
tated such  a  defensive  armament  on  the  part  of  Triceratops  and  its 
kin.  The  ceratopsians  were  strictly  North  American  and  lived  for 
only  a  brief  span,  geologically  speaking;  for  they  are  confined  exclu- 
sively to  the  Upper  Cretaceous. 

The  Extinction  of  the  Dinosaurs. — "One  of  the  most  inexpli- 
cable of  events"  says  Lull,  "is  the  dramatic  extinction  of  this  mighty 
race,  for  in  the  rocks  of  undoubted  Tertiary  age  not  a  single 
trace  of  them  remains.  One  student  has  argued  internecine  war- 


224  VERTEBRATE  ZOOLOGY 

fare  among  the  dinosaurs  themselves;  another,  the  destructive 
slaughter,  not  of  adults  but  of  young,  possibly  while  yet  in  the 
egg,  by  small  blood-thirsty  mammals;  yet  another,  change  of  climate, 
either  by  the  diminution  of  the  necessary  heat  without  which  no 
reptilian  race  may  thrive,  or  of  the  moisture  with  an  accompanying 
change  of  vegetation.  These  are  all  conjectural  causes  of  extinction; 
but  this  we  know,  that  with  the  extensive  changes  in  the  elevation  of 
land  areas  which  marked  the  close  of  the  Mesozoic,  came  the  draining 
of  the  great  inland  Cretaceous  seas  along  the  low-lying  shores  of 
which  the  dinosaurs  had  their  home,  and  with  the  consequent  re- 
striction of  old  haunts  came  the  blotting  out  of  a  heroic  race.  Their 
career  was  not  a  brief  one,  for  the  duration  of  their  recorded  evolution 
was  thrice  that  of  the  entire  mammalian  age.  They  do  not  represent 
a  futile  attempt  on  the  part  of  nature  to  people  the  world  with  crea- 
tures of  insignificant  moment,  but  are  comparable  in  majestic  rise, 
slow  culmination,  and  dramatic  fall  to  the  greatest  nations  of  antiq- 
uity." 

PTEROSAURIA. — The  pterosaurs  represent  a  series  of  experiments 
in  aviation  on  the  part  of  the  reptiles.  They  varied  greatly  in  size 
from  tiny  forms  comparable  with  our  sparrows  to  flying  dragons  with 
a  wing-spread  of  twelve  feet.  That  they  were  good  flyers,  able  to 
venture  far  out  over  the  sea,  is  indicated  by  the  fact  that  their  remains 
are  found  mingled  with  those  of  the  marine  mososaurs  miles  away 
from  the  Mesozoic  shore  lines.  They  were  scarcely  flyers  in  the 
strict  sense,  but  must  have  been  effective  gliders  or  soarers;  for  they 
did  not  possess  the  powerful  musculature  necessary  for  active  flight. 
In  Pterodon  (Fig.  124,  D),  one  of  the  largest  of  the  flying  reptiles,  the 
head  is  prolonged  into  a  great  keel-like  structure,  used  as  a  balancing 
mechanism.  Other  types  had  a  long  tail  with  a  terminal  rudder-like 
expansion  much  like  that  of  an  aeroplane. 

The  pterosaurs  or  pterodactyls  might  appear  superficially  to  be  well 
suited  to  be  the  ancestors  of  the  birds,  but  anatomically  they  are 
quite  unsuited  for  this  role.  They  represent  simply  a  highly  specialized 
adaptive  radiation,  that  was  short-lived  and  met  with  utter  extinc- 
tion during  the  Upper  Cretaceous.  They  furnish  a  final  chapter  in  the 
remarkable  adaptive  radiation  of  the  Mesozoic  reptiles 


lapter 


REPTILIA  225 

MODERN  REPTILES 

The  following  list  of  diagnostic  characters,  which  should  be  com- 
pared with  a  similar  list  already  given  for  Amphibia,  is  taken  from 
Gadow: 

CHARACTERS  OF  REPTILIA 

(D  The  vertebrae  are  gastrocentrous. 

fzJThe  skull  articulates  with  the  atlas  by  one  condyle,  which  is 
formed  mainly  by  the  basioccipital. 

3.  The  mandible  consists  of  many  pieces  and  articulates  with  the 

cranium  through  the  quadrate  bones. 

4.  There  is  an  auditory  columellar  apparatus  fitting  into  the  fe- 

nestra  ovalis. 

5.  The  limbs  are  of  the  tetrapodous  pentadactyl  type. 

6.  There  is  an  intracranial  hypoglossal  nerve. 

7.  The  ribs  form  a  true  sternum. 

8.  The  ilio-sacral  connection  is  post-acetabular. 

9.  The  skin  is  covered  (a)  with  scales,  but  (b)  neither  with  feathers 

nor  with  hairs;  and  there  is  a  great  paucity  of  glands. 
J£oReptiles  are 


11.  The  red  blood-corpuscles  are  nucleated,  biconvex  and  oval. 
12._  The  heart  is  divided  into  two  atria  and  an  incompletely  divided 

ventricle.    It  has  no  conus,  but  semilunar  valves  exist  at  the 

base  of  the  tripartite  aortic  trunk. 

13.  The  right  and  left  aortic  arches  are  c6mplete  and  remain  func- 
•—  —  •  .-""-^ 

tional.  -  Jjf*^*' 

Respiration  is  effected  by  lungs;  and.  "gills  are  entirely  absent 
even  during  embryonic  life. 


Lateral  sense  organs  are  absent. 

"he  kidneys  have  no  nephrostomes.    Each  kidney  has  one  sep- 
arate ureter. 

17.  There  is  always  a  typical  cloaca. 

18.  The  eggs  are  meroblastic. 

19.  Fertilization  is  internal,  and  is  effected,  with  the  single  exception 
/V.       of  Sphenodon,  by  means  of  copulatory  organs. 

I  20.J  An  amnion  and  an  allantois  are  formed  during  development. 
Numbers  1,  2,  3,  7,  8,  14,  16,  18,  20  separate  the  reptiles  from 
the  Amphibia. 


226  VERTEBRATE  ZOOLOGY 

Numbers  9  (b),  10,  12,  and  13  separate  them  from  the  birds  and 

mammals. 
Numbers  3,  8,  and  1 1  separate  them  from  the  mammals. 

ANATOMY  OF  A  MODERN  REPTILE  (TURTLE) 

For  several  reasons  the  turtle  or  tortoise  is  chosen  as  a  reptilian 
type  for  detailed  description  rather  than  a  lizard,  although  the  latter 
is  in  most  respects  a  more  representative  form,  and  considerably  less 
specialized  than  are  any  of  the  chelonians.  The  first  reason  for  our 
choice  is  one  of  expediency,  for  the  turtles  are  the  most  plentiful 
reptile  in  by  far  the  greater  part  of  the  United  States.  A  second  rea- 
son is  that  they  are  of  convenient  size  for  laboratory  work  and  are  of 
compact  structure.  There  is,  however,  a  third  and  more  fundamental 
advantage  gained  by  the  use  of  the  turtle;  it  is  structurally  more  nearly 
related  to  the  group  of  reptiles  from  which  the  mammals  are  believed 
to  have  arisen  than  is  any  other  living  reptile. 

EXTERNAL  CHARACTERS 

The  turtle  is  a  reptile  in  a  box.  This  box,  whether  it  forms  a  com- 
plete or  only  a  partial  housing  for  the  body,  head,  limbs  and  tail,  has 
a  dome-shaped  roof,  called  the  carapace  and  a  flat  floor  called  a  plas- 
tron. Paired  lateral  pillars  join  the  floor  to  the  roof.  The  box  is  open 
broadly  in  front  and  behind  in  order  to  allow  the  head,  legs,  and  tail 
to  emerge;  b\it  these  appendages  can  all  be  withdrawn  within  the 
shelter  of  the  eaves,  and  in  some  cases  the  front  and  rear  sections  of 
the  floor  (plastron)  are  hinged  in  such  a  way  that  they  can  bend 
upward  and  completely  close  the  house  after  the  appendages  have 
been  drawn  in.  The  head  is  of  moderate  size  and  somewhat  flat;  the 
neck  is  characteristically  long  and  flexible  and  capable  of  being 
folded  up  when  the  head  is  withdrawn;  the  mouth  is  large  and  tooth- 
less, but  is  provided  with  a  sharp-edged,  horny  beak.  The  external 
nares  (nostrils)  are  close  together  near  the  end  of  the  snout,  sometimes 
protruding  into  a  regular  proboscis.  The  eyes  are  situated  laterally 
and  have  three  eyelids:  a  short  opaque  upper  lid,  a  longer  lower  lid 
which  makes  the  turtle  shut  its  eye  upwards  instead  of  downwards  as 
a  man  does,  and  a  third  eyelid  or  transparent  nictitating  membrane 
which  may  be  drawn  across  the  eye  from  the  inner  corner.  The 
tympanic  membrane  is  quite  similar  to  that  of  the  frog  a 'id  is  just  back 
of  the  gape  of  the  jaws.  The  feet  are  pentadactyl  and  each  finger  is 


REPTILIA 


227 


usually  armed  with  a  claw.  As  a  rule  the  feet  are  webbed  as  in  aquatic 
birds.  The  skin  of  the  head  is  usually  smooth  and  scaleless,  as  is 
also  the  neck  in  most  species;  but  the  rest  of  the  body  is  usually 
covered  with  scales,  except  the  base  of  the  thighs.  The  tail  is  as  a 
rule  poorly  developed,  but  in  the  more  primitive  types,  as  for  example 
the  snapping  turtles,  it  may  retain  its  primitive  reptilian  proportions. 

THE  ARMATURE 

The  carapace  and  plastron  (Fig.  129)  are,  in  most  of  our  modern 
chelonians,  somewhat  stereotyped  structures;  they  have  settled  down 


A  B 

FIG.  129.  A,  Carapace;  B,  Plastron  of  tortoise,  Graptemys.  Capital  letters 
refer  to  chitinous  scales  or  scutes,  small  letters  to  bony  plates  whether  cartilag- 
inous or  dermal.  A6,  Abdominal  scute;  An,  Anal  scute;  Cl~4,  costal  scutes, 
cl-8,  costal  plates;  e,  epiplastral  plate,  en,  endoplastral  plate;  F,  femoral  scute; 
G,  gular  scute;  H,  humeral  scute;  ho,  hyoplastral  plate;  hp.  hypoplastral  plate; 
7,  inguinal  scute;  M,  marginal  scutes;  m,  marginal  plates;  Nl-5,  neural  scutes; 
nl-8,  neural  plates;  NU,  nuchal  scute;  pr,  1,  2,  procaudal  plates;  X,  axillary 
scute;  x,  xiphiplastral  plate.  (From  Newman.) 

upon  a  very  definite  arrangement  of  the  principal  units  of  structure. 
The  carapace  (Fig.  129,  A)  is  composed  of  two  kinds  of  bony  elements 
(dermal  and  cartilaginous)  and  corneous  scutes  or  shields.  The  main 
part  of  the  bony  carapace  is  composed  largely  of  the  much  broadened 


228  VERTEBRATE  ZOOLOGY 

tips  of  the  spinal  processes  of  the  vertebrae  and  of  the  much  flattened 
ribs;  there  are  usually  eight  neural  plates  and  eight  pairs  of  costal 
plates.  In  front  of  the  first  neural  is  a  dermal  plate,  the  nuchal; 
back  of  the  eighth  neural  are  usually  three  dermal  plates,  the  first 
and  second  procaudals  and  the  pygal.  Around  the  margin  of  the 
carapace  are  usually  eleven  pairs  of  dermal  plates,  the  marginals. 
Overlying  the  bony  carapace  there  is  a  horny  carapace  composed  of 
five  neural  scutes,  four  pairs  of  costals,  a  small  anteriorly  placed  nuchal, 
and  twelve  pairs  of  marginals.  This  elaborate  composition  prevails 
in  nearly  all  of  our  modern  turtles  as  well  as  in  many  species  long 
extinct. 

The  plastron  (Fig.  129,  B),  like  the  carapace,  is  composed  of  two 
kinds  of  bony  elements  covered  with  horny  elements.  The  bony 
elements  consist  of  four  pairs  of  plates:  the  epi-,  hyo-,  hypo-  and 
xiphi-plastrals.  The  epiplastrals  are  the  modified  clavicles,  the 
hypoplastrals  and  xiphiplastrals  are  broadened  abdominal  ribs,  the 
hyoplaslrals  appear  to  be  dermal  elements  without  homologies.  A 
small  median  dermal  element  between  the  epiplastrals  and  hyoplas- 
trals  is  called  the  endoplastrals.  There  are  usually  six  pairs  of  horny 
scutes  that  break  the  joints  of  the  bony  plastron.  The  pillars  be- 
tween the  carapace  and  plastron  are  derived  from  the  hyoplastrals 
and  hypoplastrals. 

The  conventionalized  pattern  of  bones  and  scutes  in  the  armature 
has  evidently  been  arrived  at  after  a  long  period  of  evolution.  Many 
evidences  indicate  that  the  ancestral  condition  was  much  more  plas- 
tic and  variable  and  that  there  were  originally  many  more  plates  and 
scutes  than  at  present.  By  dropping  out  both  longitudinal  and  trans- 
verse rows  of  elements  the  whole  system  has  been  greatly  simplified. 
Most  species  of  turtles  to-day  show  a  certain  percentage  of  individuals 
with  supernumerary  scutes  and  plates,  that  are  evidently  vestiges  of 
ancestral  conditions. 

The  vertebrce  in  the  trunk  region  are  rigidly  united  to  the  narrowed 
bases  of  the  paddle-like  ribs.  They  are  not  very  numerous:  8  cervi- 
cal, 10  thoracic,  2  sacral,  and  a  variable  number  of  caudal  vertebrae, 
which  are  proccelous  in  form. 

One  of  the  most  puzzling  features  of  the  skeleton  (Fig.  130)  is  the 
peculiar  position  of  the  limb  girdles.  Both  pectoral  and  pelvic  gir- 
dles are  inside  instead  of  outside  of  the  ribs.  How  they  got  inside  is 
a  mystery  that  not  even  a  study  of  their  embryogenesis  is  able  to 


REPTILIA 


229 


clear  up,  for  they  arise  from  primordia  internal  to  the  ribs.  The  pec- 
toral girdle  consists  of  a  triradiate  group  of  flattened  bones:  the 
scapula,  the  procoracoid  and  the  coracoid,  the  last  being  the  largest. 
Together  they 
unite  to  form  the 
socket  which  re- 
ceives the  head  of 
the  humerus.  The 
pelvic  arch  is 
more  compact  and 
is  composed  of  the 
pubis,  ischium  and 
ilium,  uniting  to 
form  the  aceta- 
bulum  for  the 
head  of  the  femur. 
The  skull  is 
fairly  generalized 
in  structure,  but 
has  some  special 
features.  The 
jaws  are  devoid 
of  teeth,  and  max- 
illary, premaxil- 


1  i     ,      ,  FIG. 

ntary  from 


130. — Skeleton  of  tortoise,  Cisludo  lutaria,  seen 
ic  ventral  side  with  plastron  removed  and  placed 
bones  are  covered  to  one  side.  C,  costal  plate;  Co,  coracoid;  e,  endoplastron, 
with  hard  chit-  eP>  epiplastron  (clavicle);  F,  fibula;  Fe  femur;  H,  humerus; 
i  '  ,i  Hyp,  hyoplastron;  Hpp,  hypoplastron;  II,  ilium;  Js,  is- 
18 »  chium;M,  marginal  plates;  Nu,  nuchal  plates;  Pb,  pubis; 
Pro,  procoracoid  process  of  scapula;  Py,  pygal  plates; 
R,  radius;  sc,  scapula;  T,  tibia;  U,  ulna;  Xp,  xiphiplastron. 
(From  Parker  and  Haswell,  after  Zittel.) 


inous 

that      form      the 

upper    and  lower 

members    of    the 

cutting  beak;  the  vomer  is  a  single  unpaired  median  bone;  there  are 

no  lachrymals  nor  ectopterygoids;  the  pterygoids  send  inwards  wings 

of  bone,  that,  with  the  aid  of  the  palatines,  form  a  continuous  roof 

to  the  mouth;  the  supraoccipital  is  prolonged  backwards  into  a 

large   narrow   process   upon   which   are   inserted   the  heavy  neck 

muscles.     All  of  these  bones,  even  the  quadrate,  are  firmly  united 

into  a  solid  cranium.    Further  details  of  the  skull  are  shown  in  the 

figure  (Fig.  131). 


230 


VERTEBRATE   ZOOLOGY 


The  digestive  system  varies  somewhat  in  carnivorous  and  her- 
bivorous forms,  but  in  all  turtles  is  comparatively  simple.    The  tongue 


wr 


pr-m 


*7 


FIG.  131. — Skull  of  turtle.  A,  lateral;  B,  ventral  view.  6s,  basisphenoid;  //•, 
frontal;  j,  jugal;  m,  maxilla;  ob,  basioccipital;  ol,  exoccipital;  op,  opisthotic;  os, 
supraoccipital;  pal,  palatine;  par,  parietal;  ph,  postfrontal;  prfr,  prefrontal;  pt, 
pterygoid;  prm,  premaxilla;  q,  quadrate;  qj,  quadrate  jugal;  «£,  squamosal;  v, 
vomer.  (From  Parker  and  Haswell,  after  Hoffmann.) 


is  broad  and  soft  and  cannot  be  protruded.    The  stomach  is  a  simple 
U-shaped  enlargement  of  the  alimentary  tract.    The  intestine  is  with- 


REPTILIA  231 

out  a  caecum;  it  is  clearly  divided  into  large  and  small  intestine.  The 
cloaca  is  proportionately  large. 

The  respiratory  organs  (lungs)  are  large  and  complicated.  In- 
halation and  exhalation  are  effected  partly  by  drawing  in  the  neck 
and  thrusting  it  out  again,  thus  decreasing  and  increasing  the  volume 
of  the  thoracic  cavity.  The  air  is  also  swallowed  into  the  lungs  by 
filling  and  then  emptying  the  throat. 

The  circulatory  system.  The  heart  is  very  broad  laterally, 
having  two  entirely  separate  atria  or  auricles,  and  a  ventricle  par- 
tially divided  into  two  parts  by  a  perforated  partition.  The  right 
auricle  receives  the  venous  blood  from  two  precaval  and  One  post- 
caval  veins;  the  blood  then  goes  to  the  right  half  of  the  ventricle,  and 
thence  through  the  pulmonary  artery  to  the  lungs.  From  the  lungs 
it  returns  through  the  pulmonary  veins  to  the  left  auricle,  thence  to 
the  left  ventricle,  which  pumps  it  out  through  the  paired  aortic  arches 
to  all  parts  of  the  body.  There  is  no  renal  portal  system,  but  the 
hepatic  portal  system  is  better  developed  than  in  the  Amphibia. 

The  urogenital  systems.  The  kidneys  are  metane^hric  bodies, 
which  pass  their  excretion  through  paired  ureters  directly  to  the 
cloaca,  thence  jnto  a  urinary  bladder,  which  in  turn  empties  into  the 
cloaca.  Thoanale  reproductive^  organs  consist  of  a  pair  of  testes,  a 
pair  of  much  coiled  vasa  deferentia,  through  which  the  sperm  passes 
to  the  grooved  penis,  which  is  attached  to  the  front  of  the  cloaca. 
The  female  organs  consist  of  paired  ovaries  and  large  oviducts  pro- 
vided with  albuminous  and  shell  glands.  The  eggs  when  laid  are 
covered  with  a  tough  shell  and  are  usually  buried  in  the  ground. 

The  nervous  system  shows  a  considerable  advance  over  that  of 
the  Amphibia.  The  cerebral  hemispheres  are  larger  and  the  cerebel- 
lum more  complete.  Many  other  changes  in  details  will  be  noted  on 
comparative  study.  The  eyes  are  small,  but  the  vision  is  keen;  the 
pupil  is  round  and  the  iris  unusually  dark  in  color.  The  sense  of  hear- 
ing is  not  very  acute;  the  tympanic  membrane  is  thin  and  exposed, 
and  is  connected  with  the  auditory  organ  by  a  slender  columellar  bone. 
The  sense  of  smell  is  the  keenest  of  the  senses  in  most  turtles,  both 
in  the  water  and  in  the  air.  In  correlation  with  the  keen  olfactory 
sense  the  olfactory  lobes  of  the  brain  are  highly  developed. 


232  VERTEBRATE  ZOOLOGY 


THE  FOUR  ORDERS  OF  LIVING  REPTILES 


The  order  P^osauria  is  represented  to-day  by  one  genus,  Sphen- 
odon,  placed  in  the*Tamily  Rhynchocephalia.  The  order  Chelonia  is 
represented  by  two  sub-orders  and  several  large  families.  The  order 
Crocodilia  is  to-day  a  minor  order,  represented  by  only  a  few  species 
of  large  reptiles.  The  order  Sauria  contains  a  large  proportion  of  liv- 
ing reptiles,  since  to  it  belong  the  lizards'  and  the  snakes. 

ORDER  PROSAURIA  (RHYNCHOCEPHALIA) 

The  only  representative  of  this  order  now  living  is  Sphenodon  (Hat- 
teria)  punctatum  (Fig  132.  A),  the  "tuatara"  of  the  Maories  of  New 
Zealand.  Gadow  refers  to  this  species  as  "the  last  living  witness  of 
by-gone  ages,  this  primitive,  almost  ideally  generalized  type  of  rep- 
tile, this  'living  fossil.'"  This  almost  reverential  attitude  toward  the 
antiquity  of  this  reptile  has,  however,  broken  down  through  the  dis- 
covery that  Paleohatteria,  the  extinct  type  which  was  supposed  to 
link  Sphenodon  with  the  remote  past,  is  really  more  nearly  an  an- 
cestral lizard  than  an  ancestral  Sphenodon;  a  fact  that  led  such  an 
authority  as  Williston  to  assert  that  some  of  our  modern  lizards  are 
more  primitive  than  is  Sphenodon. 

The  tuatara  is  decidedly  primitive  in  some  features.  There  is  no 
penis;  the  centra  of  the  vertebrae  are  amphicoelous;  the  first  three 
basiventralia  are  quite  large;  and  the  skeletal  structure  of  both  limbs 
and  girdles  is  primitive.  The  skull  (Fig.  132,  B,  C,  D)  is  an  almost 
ideally  generalized  reptilian  skull  and  well  repays  study. 

Sphenodon  is  facing  extinction  at  the  present  time.  Already  it  has 
disappeared  from  the  mainland  and  is  confined  to  a  few  small  islands 
off  the  coast  of  New  Zealand.  For  years  it  has  been  assiduously 
hunted  by  the  Maories  who  consider  its  flesh  an  unusual  delicacy. 
Unless  protected  it  will  soon  go  the  way  of  its  extinct  ancestors  and 
will  be  represented  only  by  a  few  specimens  in  our  museums. 

The  tuatara  is  nocturnal  in  habits,  living  in  burrows  during  the  day. 
It  shares  its  capacious  burrow  with  various  kinds  of  petrels,  with 
which  it  lives  on  apparently  amicable  terms.  It  will  not  tolerate  any 
other  species  of  guest,  not  even  other  individuals  of  the  same  species, 
and  viciously  attacks  any  invader  no  matter  how  formidable.  Its 
lizard-like  aspect  is  only  skin  deep;  for  it  requires  only  a  very  casual 


REPTILIA 


233 


study  of  the  skeleton  and  viscera  to  see  the  difference  between  this 
form  and  any  of  the  lizards.  The  illustration  (Fig.  132,  A)  shows 
better  than  a  verbal  description  its  salient  external  features. 


FIG.  132. — Sphenodon  punctatum.  A,  Lateral  view;  B,  dorsal  view  of  skull; 
C,  ventral  view  of  skull;  D,  lateral  view  of  skull,  c,  condyle;  cl,  columella; 
ep,  ectopterygoid;  /,  frontal;  j,  jugal;  m,  maxillary;  pm,  premaxillary;  n,  nasal; 
p,  parietal;  pi,  palatine;  prf,  prefrontal;  ptf,  postfrontal  and  post-orbital;  pg,  ptery- 
goid;  q,  quatrate  or  quadratoj  ugal ;  sq,  squamosal;  v,  vomer.  (After  Gadow.) 

ORDER  CHELONIA   (TURTLES  AND  TORTOISES) 

The  members  of  this  order  are  so  uniquely  modified  that  there  is 
no  difficulty  in  recognizing  them  and  in  distinguishing  them  from  all 
other  living  creatures.  Their  short,  broad  body,  covered  by  the 


234  VERTEBRATE  ZOOLOGY 

characteristic  carapace  and  plastron,  and  their  horny,  toothless  jaws, 
constitute  their  outstanding  characteristics. 

They  occupy  a  very  wide  range  of  habitat  zones  without  displaying 
any  very  radical  departures  from  the  typical  chelonian  form  and  pro- 
portions. They  range  from  the  pure  marine  types  which  come  on  land 
only  for  the  purpose  of  laying  their  eggs  in  the  sand ;  through  a  whole 
series  of  amphibious  forms,  living  in  ponds  and  spending  a  consider- 
able part  of  the  time  on  land;  culminating  in  the  giant  purely 
terrestrial  forms  that  are  found  on  several  groups  of  oceanic  islands. 
Their  adaptive  radiation  does  not  include  arboreal,  cursorial  or 
volant  types,  for  the  probable  reason  that  the  shape  and  weight 
of  the  armature  does  not  readily  lend  itself  to  these  modes 
of  life. 

Until  recently  the  ancestry  of  the  Chelonia  was  entirely  a  mystery, 
but  it  is  now  believed  by  palaeontologists  that  the  Eunotosauria  of 
Permian  times  furnish  a  connecting  link  between  the  Chelonia  and 
still  more  primitive  cotylosaurs  of  the  Permo-Carboniferous.  The 
Eunotosauria  have  been  ably  discussed  by  Watson  and  there  is  now 
very  general  agreement  with  his  contention  that  these  forms  repre- 
sent a  group  which  was  ancestral  to  the  Chelonia. 

The  order  Chelonia  is  divided  into  two  sub-orders,  the  Athecoe  (with- 
out a  true  carapace),  and  the  Thecophora  (with  a  carapace). 

SUB-ORDER  I.  ATHEC^J 

The  sole  living  representative  of  this  sub-order  is  Dermochelys 
coriacea,  the  Leather-back  Turtle  (Fig.  133,  A).  Instead  of  the  usual 
closely-knit  carapace  and  plastron  it  has  twelve  longitudinal  rows  of 
dermal  plates  (5  dorsal,  5  ventral  and  2  lateral).  The  homologues  of 
these  can  be  recognized  in  the  scute  rows  of  some  of  the  Thecophora. 
The  limbs  are  large,  flipper-like  paddles  of  a  highly  specialized  aquatic 
type.  The  tail  is  rudimentary.  Dermochelys  has  a  wide  distribution, 
ranging  over  all  of  the  inter-tropical  seas,  but  is  nowhere  abundant. 
It  is  carnivorous,  feeding  chiefly  on  mollusks,  fishes  and  crustaceans. 
One  of  the  most  peculiar  facts  about  this  species  is  that  only  large 
specimens  and  "babies"  have  ever  been  found.  Where  they  pass  the 
many  years  of  their  youth  and  early  maturity  is  a  mystery.  Possibly 
there  is  in  some  obscure  corner  of  the  world  an  undiscovered  Der- 
mochelys rookery. 


REPTILIA 


235 


It  is  believed  by  Dollo  that  Dermochelys  is  derived  from  an  early 
terrestrial  thecophoran,  which  lost  the  primitive  armature  when  it 
assumed  the  completely  marine  habit;  a  belief  that  involves  the  idea 
of  reversed  aquatic  adaptation.  Structurally  the  " leather-backs" 
are  so  different  from  the  other  turtles  that  some  authors  advocate 


FIG.  133. — Group  of  Chelonia,  I.  A,  Leatherback  Turtle,  Dermochelys  (Sphar- 
gis)  coriacea;  B,  Hawksbill  Turtle,  Chelone  imbricata;  C,  Chelydra  serpenlina 
(Snapping  Turtle) ;  D,  Pennsylvania  Mud  Turtle,  Cinosternum  pennsylvanicum  ; 
E,  European  Pond-Tortoise,  Emys  orbicularis;  F,  Carolina  Box-Tortoise,  Cistudo 
(Terrapene)  Carolina.  (Redrawn  after  Lydekker.) 

putting  them  in  a  separate  order.  Recent  discoveries,  however, 
have  tended  to  confirm  the  conviction  that  they  are  an  early  aberrant 
offshoot  of  a  primitive  land  chelonian  stock. 

SUB-ORDER  II.  THECOPHORA  (TRUE  TURTLES) 

The  true  turtles  are  subdivided  into  two  assemblages:  Cryptodira 
and  Pleurodira. 


236  VERTEBRATE  ZOOLOGY 


DIVISION  1.  CRYPTODIRA 

In  these  turtles  the  carapace  is  covered  with  horny  scutes:  the 
neck  is  retractile,  bending  chiefly  in  a  vertical  plane;  and  the  pelvis 
is  not  fused  with  the  shell.  By  far  the  majority  of  our  common 
turtles  and  tortoises  belong  to  this  division. 

Family  1.  Chelydridae  (Snapping  Turtles) — The  common  snapper 
(Chelydra  serpentina)  and  the  alligator  snapper  (Macrochelys 
temmincki),  both  North  American  species,  are  the  only  living  rep- 
resentatives of  this  primitive  family.  The  common  snapper 
(Fig.  133,  C)  is  our  most  generalized  modern  turtle.  Its  head, 
body  and  tail  are  rather  evenly  balanced,  and  the  limbs  are  propor- 
tionally heavy  and  typically  reptilian.  There  is  also  less  complete 
boxing  in  of  the  movable  parts  than  in  most  other  species.  In  the 
tail  of  Chelydra  are  found  not  only  the  rows  of  plates  and  scutes  that 
are  homologous  with  those  in  the  armature,  but  at  least  five  rows  that 
have  disappeared  from  or  are  merely  vestigial  in  the  latter.  Hence 
the  ancestral  condition  of  the  armature  is  probably  more  nearly  du- 
plicated in  the  basal  portion  of  the  tail  of  Chelydra  than  anywhere  else. 
The  snapper  is  a  slow  and  clumsy  creature,  exceedingly  sullen  and  ill- 
tempered  in  captivity.  When  irritated  it  snaps  blindly  with  widely 
open  mouth,  and  seizes  indiscriminately  any  object  within  reach.  It  is 
decidedly  aquatic  in  habit  and  is  not  fond  of  basking  in  the  open. 
More  of  ten  it  is  found  in  shallow,  warm  pools  partly  buried  in  the  mud. 
At  times  it  goes  on  journeys  cross-country  from  one  body  of  water  to 
another.  The  snapper  makes  its  nest  in  loose  gravelly  or  sandy  soil  at 
no  great  distance  from  the  water's  edge,  though  it  may  wander  some 
distance  inland  before  selecting  a  suitable  nesting  place.  In  excavating 
the  nest  a  shallow,  funnel-like  depression  is  first  made;  then  a  crude 
tunnel  is  scraped  out  and  enlarged  at  the  bottom  into  a  chamber. 
All  of  the  digging  is  done  with  the  hind  feet,  which  are  armed  with 
heavy  claws.  About  thirty  to  forty  spherical  eggs  with  tough  elastic 
shells  are  laid  layer  on  layer  with  pads  of  sand  packed  between;  and 
a  layer  of  sand  is  packed  in  and  smoothed  over  the  top.  Chelydra 
is  carnivorous,  feeding  on  fish,  frogs,  young  ducks  and  all  other 
aquatic  animals  that  come  its  way.  Active  prey  is  caught  by 
stealth.  The  dull,  mud-colored  body  renders  it  inconspicuous  and 
aids  it  in  slipping  up  close  to  an  unwary  frog  or  fish.  If  the  snapper 
ever  approaches  a  prospective  victim  so  as  to  be  able  to  snap  its 


REPTILIA  237 

jaws  upon  it,  the  victim  is  doomed,  for  once  closed  the  jaws  are  like 
a  steel  trap. 

While  the  ordinary  snapper  may  reach  a  weight  of  twenty  pounds 
the  alligator  snapper  grows  to  twice  that  weight  or  more  and  is 
proportionately  more  ferocious.  It  is  said  that  a  large  specimen  is 
capable  of  biting  off  a  piece  of  board  over  an  inch  in  thickness. 

Family  2.  Dermatemydae.— This  is  a  small  group  of  mostly 
Central  American  tortoises,  with  a  strictly  aquatic  habitat.  They  are 
primitive  in  having  a  row  of  scutes  between  the  marginals  and  plas- 
trals,  called  inframarginals.  This  row  is  represented  by  the  merest 
vestiges  in  other  families  of  Chelonia. 

Family   3.  Cinosternidae    (Skunk    or    Musk    Turtles). — This   is 
another  group  that  is  primarily  aquatic,  but  not  so  exclusively  so  as 
the  first  two  families  described.    They  are  small  turtles  (Fig.  133,  D) 
that  show  in  their  structures  evidence  of  having  reverted  to  an  aquatic 
abode  after  a  prolonged  ancestry  upon  the  land.    Their  box-like  shell 
is  not  the  type  of  armature  that  is  characteristic  of  the  really  aquatic 
turtles.     The  commonest  representative  of  the  group  is  Aromochelys 
odorata,  a  name  redolent  of  the  peculiar  sickening  odor  that  it  gives 
off  from  the  inguinal  glands  when  disturbed.    The  "  stink  pot,"  as  it 
is  commonly  called,  lives  at  the  bottom  of  ponds,  crawling  over  the 
mud,  but  seldom  swimming  freely  in  the  water.    Its  heavy  shell  makes 
it  a  sort  of  chelonian  diver.    In  warm  weather  it  is  often  seen  floating 
at  the  surface  supported  upon  a  mass  of  floating  pond-scum.    They 
rarely  bask  openly  above  the  water.    On  land  they  are  slow  and  clumsy 
of  gait,  but  in  spite  of  this  they  wander  about  at  night  through  the 
grass  and  shore  herbage,  hunting  for  worms  and  slugs.    I  have  also 
found  them  in  the  daytime  rooting  about  in  the  moss  for  insects  or 
grubs,  using  their  snouts  for  the  purpose  and  snuffing  like  little  pigs. 
Sometimes  they  stay  out  of  water  so  long  that  they  become  light  in 
weight  from  desiccation.     When  caught  they  make  a  great  show  of 
fierceness,  hissing  and  opening  the  jaws  widely,  looking  almost  as 
formidable  as  a  small  snapper;  but  this  is  either  a  mere  "bluff"  or 
due  to  fright,  for  when  given  the  opportunity  to  bite  they  do  not  take 
advantage  of  it.     Their  appetite  is  insatiable  and  indiscriminate; 
anything  that  could  by  any  stretch  of  courtesy  be  described  as  edible 
meets  with  their  approval.    Aromochelys  is  a  curious  mixture  of  a 
primitive  and  specialized  turtle.    It  is  very  aquatic  at  certain  times 
and  decidedly  terrestrial  at  others.     It  pretends  to  be  fierce,  but  i& 


238  VERTEBRATE  ZOOLOGY 

gentle;  it  is  omnivorous.  It  makes  the  crudest  nest  of  any  of  the 
species  that  the  writer  has  studied.  On  one  occasion  a  female  was 
observed  to  dig  a  shallow  hole  about  two  inches  wide  and  about 
as  deep.  Two  china-like  eggs  were  laid  in  the  nest  and  covered  up 
loosely  with  debris.  Sometimes  the  nest  is  constructed  with  some- 
what greater  care,  but  it  is  less  elaborate  than  in  other  species 
studied. 

Family  4.  Platysternidae. — This  family  is  represented  by  one 
species  native  to  Borneo,  Siam,  and  Southern  China.  Platysternum 
is  an  extremely  flat  type,  with  unusually  large  head  and  hooked 
beak. 

Family  5.  Testudinidse  (the  common  pond  tortoises). — This  is 
much  the  largest  family  of  chelonians  and  is  represented  in  North 
America  by  Graptemys  geographica  (the  map  tortoise),  Chrysemys 
picta  (the  painted  tortoise),  Nannemys  gutatta  (the  spotted  tortoise), 
Terrapene  Carolina  (the  box  tortoise)  and,  as  an  aberrant  derivative  of 
North  American  chelonians,  the  giant  land  tortoises  of  the  Galapagos 
and  other  oceanic  islands.  In  habits  they  range  from  aquatic  to 
purety  terrestrial  forms.  Some  are  purely  carnivorous,  other  purely 
herbivorous. 

Perhaps  the  commonest  example  of  our  pond  tortoises  is  Chry- 
semys picta  (eastern  variety)  or  C.  marginata  (western  variety).  These 
rather  small  tortoises  are  found  in  ponds  or  sluggish  streams.  They 
are  most  frequently  seen  when  basking  in  the  sun  along  the  shore  or 
upon  floating  logs.  They  are  excellent  swimmers  and  somewhat 
difficult  to  catch.  They  feed  upon  dead  fish  and  other  carrion  in  the 
water,  tearing  up  the  flesh  with  their  long,  sharp  claws  and  sharp- 
edged  beaks.  The  nest  is  made  with  a  narrow  neck  and  a  flask-shaped 
chamber  at  the  bottom.  It  is  situated  in  moist  sand  along  the  shores 
of  still  waters.  Four  to  eight  oval  eggs  are  laid;  these  are  placed  in 
the  flask-like  enlargement  and  are  covered  up  neatly  with  sand,  which 
is  pounded  down  with  the  knuckles  of  the  hind  feet.  Chrysemys  is  a 
bright,  intelligent  little  tortoise,  showing  little  sullenness  when  cap- 
tured, and  no  disposition  to  snap  or  to  take  alarm.  They  soon  learn 
to  come  to  one  who  habitually  feeds  them  and  will  eat  from  the 
hand. 

Terrapene  Carolina  (Fig.  133,  F)  is  the  common  land  terrapin  of  the 
Southern  and  Eastern  States.  Structurally  they  differ  little  from 
some  of  the  pond  tortoises,  but  they  have  acquired  exclusively  terres- 


REPTILIA  23ft 

trial  habits.  If  put  in  the  water  they  soon  drown.  They  are,  like  the 
pond  tortoises  and  unlike  the  giant  land  tortoises,  largely  carnivorous. 
In  captivity  they  become  very  tame  and  are  often  used  as  pets.  There 
are  records  of  individuals  having  lived  in  captivity  for  fifty  years  or 
more.  They  bask  in  the  heat  of  the  sun  most  of  the  day,  but  at  dusk 
they  become  active,  hunting  for  slugs  and  worms,  which  form  their 
chief  diet.  At  night  they  retire  to  their  burrows.  Their  nesting  habits 
are  much  like  those  of  the  pond  tortoises. 

The  true  land  tortoises  range  from  forms  of  moderate  size,  like  Tes- 
tudo  grceca,  the  common  European  species,  to  the  giant  land  tortoises 
of  the  oceanic  islands  (Fig.  134,  C).  These  creatures  do  not  differ 
materially  from  others  except  in  size,  a  character  which  may  have 
been  the  result  of  the  easy  conditions  of  life  on  oceanic  islands  or  it 
may  be  merely  one  of  the  effects  of  senescence.  They  are  herbivorous 
and  devour  quantities  of  young  plant  shoots  and  other  succulent 
vegetation.  In  the  Galapagos  Islands  there  is  a  different  species  of 
land  tortoise  for  almost  every  island.  It  is  believed  that  the  first  in- 
dividual or  pair  of  these  animals  reached  the  Galapagos  land  mass 
when  it  was  a  single  small  continent,  that  subsidence  of  that  part  of 
the  earth's  crust  left  only  the  high  places  above  water,  and  that  these 
are  the  present  islands.  Isolation  of  the  tortoises  on  the  different 
islands  is  supposed  to  have  been  the  principal  agency  in  establishing 
different  species  on  the  various  islands.  The  largest  specimens  of  land 
tortoises  weigh  over  five  hundred  pounds  and  are  over  four  feet  in 
length  of  shell.  They  are  said  to  exhibit  remarkable  longevity,  some 
having  a  record  of  about  one  hundred  and  fifty  years. 

Family  6.  Chelonidae  (Sea  Turtles). — This  group  is  best  known 
for  the  so-called  tortoise-shell,  a  product  derived  from  the  horny 
scutes  with  which  the  carapace  is  covered.  They  are  large  turtles 
with  paddle-like  limbs,  small  head,  short  neck  and  rudimentary  tail. 
They  come  ashore  only  to  lay  their  eggs  in  the  beach  sand  of  the  trop- 
ical sea-shores.  At  that  time  they  are  captured  in  large  numbers  and 
brought  to  the  metropolitan  markets,  where  their  flesh  meets  with  a 
ready  sale  as  a  material  for  soup.  When  they  are  out  of  the  water  they 
are  very  clumsy  and  are  easily  caught.  All  that  the  hunter  has  to  do 
to  capture  his  prey  is  to  turn  it  over  on  its  back,  where  it  is  safe  until 
such  time  as  it  is  convenient  to  load  it  into  boats.  Usually  they  are 
kept  in  water-filled  inclosures  till  needed  and  shipped  alive  to  the 
market. 


240 


VERTEBRATE  ZOOLOGY 


Chelone  mydas,  the  green  turtle,  is  the  largest  and  best  known  species 
of  sea  turtle,  though  the  "hawksbill,"  Chelone  imbricata  (Fig.  134,  B) 
is  a  close  rival  in  popularity.  Thallasochelys,  "the  loggerhead  turtle," 
though  it  is  of  no  commercial  value  on  account  of  its  rank  flesh,  is  of 


FIG.  134. — Group  of  Chelonia,  II.  A,  Snake-necked  Tortoise,  Hydromedvsa 
maximiliani;  B,  Soft-shelled  Tortoise,  Aspidonectes  spinifer;  C,  Giant  Land 
Tortoise  or  Elephant  Tortoise,  Testudo  elephaniina.  D,  The  "Matamata," 
Chelys  fimbriata.  (All  redrawn,  A  and  C  after  Lydekker,  B  and  D,  after  Gadow.) 

considerable  interest  on  account  of  the  fact  that  it  exhibits  a  remark- 
able diversity  of  scute  and  plate  number  and  arrangement.  This  is 
probably  an  evidence  of  primitiveness,  and  may  approach  the  an- 
cestral condition. 


REPTILIA  241 

DIVISION  2.  PLEURODIRA 

The  Pleurodira  play  the  same  role  in  the  southern  hemisphere  that 
is  played  by  the  Testudinidse  in  the  northern  regions.  They  are  less 
diversified,  however,  than  the  northern  tortoises  in  that  they  are  all 
aquatic.  They  differ  from  our  tortoises  mainly  in  that  the  neck,  in- 
stead of  being  withdrawn  within  the  carapace  between  the  shoulders, 
is  bent  laterally  and  tucked  under  the  edge  of  the  shell  on  one  side. 
The  pelvic  girdle,  unlike  that  of  our  tortoises,  is  fused  to  the  tail  and 
to  the  carapace. 

The  genus  Chelodina  will  serve  as  an  example  of  these  southern 
tortoises.  The  carapace  is  much  like  that  of  Chrysemys,  but  the  plas- 
tron has  a  novel  feature  in  the  form  of  a  small  median  scute,  the  inter- 
plastral,  which  is  believed  to  be  a  vestige  of  an  ancestral  row  of  scutes 
that  has  been  lost  by  most  turtles.  They  are  good  swimmers  and  feed 
exclusively  upon  aquatic  animals  such  as  frogs  and  water  insects. 
The  long  neck  undulates  from  side  to  side  like  that  of  a  snake.  When 
basking  they  tuck  the  head  away  under  the  shell  in  the  manner  de- 
scribed. There  seems  to  be  no  striking  difference  between  these  tor- 
toises and  our  own  with  respect  to  breeding  and  nest-making  habits. 
The  Snake-Necked  Turtle,  Hydromedusa  maximiliani  (Fig.  134,  A)  is 
another  familiar  example  of  this  sub-order. 

TRIONYCHID.E  (SOFT-SHELLED  TORTOISES) 

The  distinguishing  character  of  these  tortoises  is  their  lack  of  the 
scaly  or  chitinous  armature.  They  also  lack  parts  of  the  bony  arma- 
ture possessed  by  other  groups.  All  over  the  body  there  is  a  reduction 
of  the  scaly  elements;  on  the  feet  the  scales  are  reduced  to  soft  folds  of 
skin.  The  "soft  shells"  are,  however,  not  to  be  pitied  for  their  de- 
fenseless state,  for  they  make  up  for  their  loss  by  their  greatly  in- 
creased intelligence  and  rapidity  of  locomotion.  Aspidonectes  spinifer 
(Fig.  134,  B)  is  the  common  "soft  shell"  of  the  Mississippi  basin 
and  is  familiar  to  most  residents  of  that  region.  Of  all  our  tor- 
toises they  are  the  most  exclusively  aquatic,  coming  inshore  only 
for  nesting  purposes,  and  seldom  basking  except  upon  floating  logs 
and  upon  low  river  banks  very  close  to  the  water's  edge.  They 
always  turn  around  after  crawling  out  of  the  water,  so  as  to  have 
the  head  turned  toward  the  water,  ready  to  scramble  into  the  river 
again  at  the  slighest  suggestion  of  danger.  They  have  need  to 


242  VERTEBRATE  ZOOLOGY 

be  wary,  for  they  are  excellent  food  for  both  man  and  beast.  I 
have  frequently  seen  young  specimens  lying  in  shallow  water  with 
only  the  proboscis-like  snout  and  the  dorsally  placed  eyes  protrud- 
ing above  the  surface.  The  body  is  usually  covered  over  with  a 
film  of  mud  which  has  been  thrown  up  by  rocking  the  body  from 
side  to  side  and  allowing  the  sediment  to  settle.  When  thus  cam- 
ouflaged they  are  reasonably  safe  from  their  enemies.  But  so  swift 
and  alert  are  the  adults  that  it  is  unlikely  that  they  would  be  caught 
by  any  of  the  creatures  that  inhabit  their  native  waters.  Even  man 
with  all  his  equipment  for  catching  animals  has  the  greatest  difficulty 
in  securing  these  tortoises.  When  one  happens  to  be  caught,  however, 
it  " keeps  its  wits  about  it,"  as  my  assistant  once  said,  and  is  ever  on 
the  alert  to  escape.  The  captor  must  be  equally  wary,  for  the  long 
neck  and  strong  jaws  have  an  unerring  aim  quite  in  contrast  with  the 
blind,  furious  lunge  of  the  " Snapper."  The  food  of  the  "soft  shell" 
consists  chiefly  of  crayfish  and  insect  larvae,  which  they  swallow  whole 
without  rending  in  pieces.  The  nest  of  this  species  is  a  rather  deep, 
neatly  made,  flask-shaped  cavity  dug  in  clean,  moist  sand.  The  fe- 
male comes  ashore  with  the  greatest  caution,  usually  very  early  in  the 
morning,  and  while  making  the  nest  stretches  the  head  on  high  on 
the  lookout  for  danger.  There  are  from  15  to  25  spherical  tough- 
shelled  eggs,  placed  in  several  layers,  with  sand  pads  between.  The 
completed  nest  is  covered  over  so  neatly  that  no  trace  of  it  is  to  be 
seen  from  the  surface.  All  of  the  activities  of  this  species  of  tortoise 
appear  to  the  writer  to  indicate  a  considerably  higher  order  of  intel- 
ligence that  that  shown  by  any  other  chelonian. 

ORDER  CROCODILIA 

This  order  is  characterized  by  its  well-proportioned  body  form, 
with  long  tail  and  well-developed  fore  and  hind  limbs;  fixed  quadrate 
bone;  teeth  fixed  separately  in  alveoli.  These  characters  apply  not 
so  well  to  ancestral  groups  of  crocodiles,  such  as  the  Pseudosuchia  and 
Eosuchia,  as  they  do  to  the  modern  types,  the  Eusuchia.  It  is  believed 
that  the  true  crocodiles  have  been  derived  from  a  generalized  diap- 
sidian  stock  as  far  back  as  the  middle  of  the  Triassic,  and  we  find 
true  fossil  crocodiles  during  the  late  Jurassic  and  a  continuous  line 
of  them  up  to  the  present. 

Structural  Characters. — The  exoskeleton  is  composed  of  squarish 
corneous  thickenings,  with  narrow  channels  of  flexible  skin  separat- 


REPTILIA 


243 


ing  the  islands  of  hard  horn.  On  the  back  and  down  the  tail  the 
scales  are  supported  by  bony  cores,  and  the  principal  scale  rows 
are  keeled,  giving  a  ridged  effect  to  the  middle  of  the  back  and  tail. 
The  hide  is  used  extensively  in  commerce. 

The  tongue  is  flat  and  thick  and  incapable  of  protrusion.  The 
lungs  are  large  and  better  developed  than  in  other  reptiles.  The 
feeih  are  large  and  formi- 
dable and^very  irregu- 
larly arranged?  Though 
the  mouth  is  provided 
with  very  powerful  mus- 
cles for  closing  the  jaws, 
those  for  opening  them 
are  very  weak,  so  that 
a  man  can  easily  close 
with  his  hands  and  keep 
closed  the  jaws  of  a  large 
specimen.  The  heart 
(Fig.  135)  and  vascular 
system  is  more  advanced 
in  the  crocodiles  than  in 
any  other  living  reptiles, 
for  the  ventricle  is 
almost  completely  di- 
vided by  a  septum  into 
a  right  and  a  left  cham- 
ber, leaving  only  a 
small  foramen  between. 
Thus  there  is  practically 
a  complete  separation 
of  venous  and  arterial  blood,  as  in  the  warm-blooded  vertebrates.. 
Though  both  right  and  left  aortic  arches  are  functional,  the  left 
arch  as  relatively  somewhat  reducedv 

The  brain  (Fig.  136)  is  decidedly  advanced  in  structure  for  a  rep- 
tilian brain,  the  large  cerebral  hemispheres  being  especially  note- 
worthy. The  tympanic  membrane  is  sunk  in  a  pit,  a  tendency  that  is 
carried  much  further  in  the  birds  and  mammals.  It  will  thus  be 
seen  that  the  crocodiles  have  followed  part  way  several  of  the  evolu- 
tionary paths  that  have  been  carried  out  fully  by  the  birds. 


FIG.  135. — Heart  of  Crocodile  with  the  principal 
arteries  (diagrammatic).  The  arrows  show  the 
direction  of  arterial  and  venous  currents.  I.  aort, 
left  aortic  arch;  I.  am,  left  auricle;  L  aur.  vent. 
ap,  left  auriculo- ventricular  aperture;  L  car,  left 
carotid;  L  sub,  left  subclavian;  I.  vent,  left  ventri- 
cle; pul.  art,  pulmonary  artery;  r.  aort,  right 
aortic  arch;  r.  aur,  right  auricle;  i.  aur.  vent,  ap, 
right  auriculo- ventricular  aperture;  r.  car,  right 
carotid;  r.  sub,  right  subclavian;  r.  vent,  right  ven- 
tricle. (From  Parker  and  Haswell,  after  Hert- 
wig.) 


244 


VERTEBRATE  ZOOLOGY 


The  geographic  distribution  of  the  crocodiles  is  wide,  but  con- 
fined chiefly  to  the  tropical  regions.  They  are  found  over  a  large 

part  of  Africa,  in  India,  Southern 
China,  Malaysia,  South  and  Cen- 
tral America,  and  along  the  Gulf 
of  Mexico  in  North  America. 
Formerly  they  occurred  in  Europe 
and  Northern  Asia. 

Habits. — The  crocodiles,  alli- 
gators and  gavials  are  all  fierce 
predaceous  creatures,  most  of 
them  being  enemies  of  both  man 
and  beast  wherever  they  grow. 
The  older  they  become  the  more 
wily  and  dangerous  they  live 
and  the  more  apt  to  become 
man-hunters.  Their  rusty,  bark- 
like  backs  give  them  the  ap- 
pearance of  partly  sunken  logs 
and  many  an  unwary  creature 
attempting  to  gain  support  upon 
such  a  "log"  has  suffered  a  rude 
awakening.  The  eggs  are  laid  in 
the  sand  much  after  the  fashion 
of  turtles.  They  reach  a  great 
age,  probably  often  breaking  over 
the  century  mark. 

Living  Crocodilia  belong  to  two 
families:  Gavialidse  and  Croco- 
dilidae. 

Family  Gavialidae. — There  is 
but  one  living  species  of  gavials, 
Gavialis  gangeticus  (Fig.  137,  C), 
confined  to  the  Ganges  and  other 
large  rivers  of  India.  They  reach 
a  length  of  over  twenty  feet,  but 


FIG.  136.— Brain  of  Alligator,  from 
above.  B.  ol,  olfactory  bulb;  G.  p, 
epiphysis;  H.  H,  cerebellum;  Med, 
spinal  cord;  M.  H,  optic  lobes;  N.  H, 
medulla  oblongata;  V.  H,  cerebral 
hemispheres;  I-XI,  cerebral  or 
cranial  nerves;  1,  2,  first  and  second 
spinal  nerves.  (From  Wiedersheim.) 


are  less  dangerous  to  man  than  are  the  true  crocodiles,  although  they 
are  believed  to  be  ever  on  the  alert  to  capture  man.  As  a  matter  of 
fact  it  is  stated  by  competent  authorities  that  they  never  attack  man, 


REPTILIA 


245 


They  differ  from  the  other  Crocodilia 


but  feed  entirely  upon  fish, 
in  that  they  have  an 
extremely  long,  nar- 
row snout,  which  re- 
sembles that  of  a 
gar-pike.  Little  is 
known  as  to  the  hab- 
its of  the  ga vials. 

Family  Crocodil- 
idae. — This  group  in- 
cludes the  old  world 
crocodiles  and  old 
and  new  world  alli- 
gators. 

The  common 
American  alligator 
(Fig.  137,  A),  Alli- 
gator mississippien- 
sis,  occurs  largely  in 
the  southeastern 
States,  living  in  the 
smaller  streams  and 
ponds.  They  usually 
lie  in  shallow  water 
with  only  the  eyes 
and  the  nostrils  ex- 
posed. When  bask- 
ing on  the  shore  and 
disturbed  by  enemies 
they  take  to  the 
water  and  quickly 
seek  the  bottom, 
where  they  bury 

themselves  in  the  FlG  i37._Group  of  Crocodilia.  A,  Alligator  missis- 
mud,  from  whence  it  sippiensis;  B,  Crocodilus  amencanus;  C,  Gavialis  gan- 
is  difficult  to  dislodge  9eticus-  (Redrawn,  A  and  B,  after  Ditmars,  C,  after 

mi  ,    Lydekker). 

them.    They  are  not 

as  large  as  the  largest  crocodiles,  reaching  a  length  hardly  over 
twelve  feet.  The  female  digs  a  large  nest  in  the  humus  and  dead 


246  VERTEBRATE  ZOOLOGY 

leaves,  which  are  piled  up  into  a  mound  and  then  hollowed  out  into 
a  receptacle  not  unlike  a  huge  bird 's  nest.  The  eggs  are  about  three 
inches  in  length  and  of  an  oval  shape  and  are  laid  to  the  number  of 
twenty  to  thirty  to  a  nest. 

The  most  typical  crocodile  is  the  classic  Crocodilus  niloticus  (Fig. 
137,  B),  the  Nile  crocodile,  which  is  believed  to  be  the  " leviathan"  of 
the  Book  of  Job.  The  armor  is  exceedingly  heavy  and  impenetrable 
to  any  weapons  but  bullets.  The  crocodile  makes  a  long  tunnel-like 
burrow  thirty  to  forty  feet  in  length,  with  an  opening  below  the  water 
level,  used  as  an  entrance,  and  with  a  large  chamber  at  the  inner  end 
well  above  the  water  level.  The  nest  is  large  and  flask-shaped  like 
that  of  some  tortoises,  but  with  a  flat  bottom  grooved  around  the 
periphery,  causing  the  eggs  to  lie  in  a  circular  ring.  The  mother  lies 
over  the  covered-up  nest  and  takes  considerable  care  of  the  young 
after  they  have  hatched. 

ORDER  SAURIA   (SQUAMATA)  LIZARDS  AND  SNAKES 

The  lizards  and  snakes  to-day  are  playing  much  the  same  role  for 
the  reptiles  that  is  played  by  the  frogs  and  toads  for  the  Amphibia. 
They  represent  climax  conditions  and  exhibit  very  pronounced 
adaptive  radiation,  being  in  both  groups  terrestrial,  fossorial,  arboreal, 
amphibious,  and  aquatic. 

They  are  characterized  by  the  possession  of:  a  movable  quadrate 
bone,  which  enables  the  mouth  to  open  more  widely;  a  transverse 
cloacal  aperture;  and  double  copulatory  organs. 

Contrary  to  the  generally  accepted  idea  that  they  have  arisen 
from  some  ancestral  prosaurian  like  Sphenodon,  they  are  now  traced 
back  to  the  early  lizard-like  group  represented  by  Varanops  (Fig. 
123,  A)  of  the  Permo-Carboniferous.  This  ancestral  type  has  been 
called  by  some  authors  Proterosauria  and  was  probably  ancestral  to 
all  of  the  Sauria  or  Squamata,  past  and  present. 

DIVISION  I.  LACERTILIA  (LIZARDS) 

It  now  appears  that  the  lizards  have  stolen  the  laurels  of  Spheno- 
don, the  reputed  prototype  of  all  the  reptiles;  for  the  earliest  known 
reptile  of  the  Permo-Carboniferous  was  a  very  generalized  and  de- 
cidedly lizard-like  creature.  Some  of  the  modern  lizards  have  de- 
parted very  little  from  that  type.  Perhaps  this  is  the  secret  of  their 


REPTILIA  247 

success  in  outlasting  the  great  reptilian  orders  that  have  come  and 
gone;  in  that  the  generalized  types  that  do  not  go  to  excesses  of  spe- 
cialization are  able  to  weather  the  age-long  vicissitudes  of  world 
change,  adapting  themselves  to  new  conditions  and  always  plastic 
enough  to  adjust  themselves  to  a  new  environment. 

Characters  of  Lacertilia. — It  is  not  so  simple  a  matter  as  one 
might  think  to  set  up  distinctions  between  lizards  and  snakes.  One 
might  think  that  the  presence  of  legs  in  the  lizards  and  their  absence 
in  snakes  would  readily  separate  the  two  groups;  but  there  are  limb- 
less lizards  and  there  are  snakes  with  at  least  rudimentary  legs.  The 
vast  majority  of  lizards,  however,  have  well-developed  legs,  only  a 
few  degraded  burrowing  forms  being  limbless.  The  lizards  also  have 
no  elastic  ligament  between  the  two  halves  of  the  lower  jaw  as  in  the 
snakes.  The  ventral  scales  are  usually  smaller  than  the  dorsal. 

The  Lacertilia  may  be  divided  into  three  sub-orders:  Geckones, 
Lacertae  and  Chamaeleontes. 

Sub-order  1.  Geckones. — The  geckos  (Fig.  138,  A)  are  primitive 
lizards  with  the  following  peculiarities:  four-footed;  amphiccelous 
vertebrae;  no  bony  temporal  arches;  dilated  clavicles;  separate 
parietals;  eyes  with  movable  lids;  tongue  broad,  fleshy,  protrusible 
and  nicked  on  the  end. 

The  geckos  are  practically  cosmopolitan  within  the  warm  temper- 
ate countries.  In  the  United  States  they  are  confined  to  our  south- 
western Pacific  regions.  They  are  wonderful  climbers.  By  means  of 
adhesive  pads  on  the  toes  they  are  able  to  ascend  the  smoothest 
surfaces  such  as  walls,  ceilings,  or  even  window-panes.  Adhesion  is 
accomplished  by  the  vacuum-cup  principle,  but  the  "cup"  consists 
of  a  complicated  system  of  lamellae.  They  feed  on  all  sorts  of  small 
animals,  especially  insects  and  spiders.  They  are  absolutely  harmless 
to  man  in  spite  of  an  undeserved  reputation  for  venomousness. 
Their  chief  defense  consists  of  an  extremely  loosely  articulated  tail, 
which  comes  off  with  great  readiness  when  seized.  When  cornered 
and  in  grave  danger  they  wag  the  tail  over  the  body,  appearing  to 
offer  it  for  seizure.  The  enemy  is  usually  satisfied  and  the  tailless 
gecko  proceeds  to  regenerate  another  tail,  which  fortunately  it  is  able 
to  do  very  readily. 

Sub-order  2.  Lacertae. — To  this  group  belong  the  great  majority 
of  modern  lizards.  As  many  as  eighteen  families  of  Lacertee  are  dis- 
tinguished by  the  authorities,  but  in  a  volume  of  the  present  scope  it 


248 


VERTEBRATE  ZOOLOGY 


would  be  unprofitable  to  deal  with  more  than  a  few  of  the  most  sig- 
nificant types. 


FIG.  138. — Group  of  Lacertilia,  I.  A,  Wall  Gecko,  Tarentola  mauritanica, 
B,  Laceita  viridis;  C,  Draco  volans  (Flying  Dragon);  D,  Iguana  tuberculata, 
E,  Sceloporus  spinosus;  F,  Helmeted  Basilisk,  Basiliscus  americanus.  (All  re- 
drawn, A,  B,  and  F,  after  Lydekker);  C  and  D,  after  Gadow;  E,  after  Ditmars.) 


REPTILIA  249 

The  members  of  this  sub-order  are  distinguished  from  the  other 
sub-orders  of  lizards  by  the  fact  that  the  vertebrae  are  procoelous  and 
solid,  and  that  the  ventral  portions  of  the  clavicles  are  not  dilated. 

It  is  proposed  to  describe  the  characters  of  a  few  well-known  species 
with  particular  reference  to  their  special  adaptive  features:  a  curso- 
rial type,  an  arboreal  type,  a  volant  type,  an  aquatic  type,  a  fossorial 
type  and  an  ant-eating  type. 

Lacerta  viridis  (Fig.  138,  B)  the  common  European  "wall  lizard"  is 
an  excellent  example  of  generalized  lizard.  It  is  a  small  type  with 
long  slender  proportions,  is  a  beautiful  green  above  and  yellow  below. 
It  runs  very  swiftly  upon  the  ground  and  over  rocks  and  hides  in  thick- 
ets and  under  any  available  shelter.'  From  some  such  generalized 
type  as  this  have  radiated  all  of  the  more  specialized  types. 

Sceloporus  spinosus  (Fig.  138,  E),  one  of  the  commonest  American 
lizards,  is  a  good  example  of  an  arboreal  type,  though  it  also  has  a 
strong  liking  for  the  ground  if  there  are  thickets  available.  It  is  a 
rusty-colored  lizard,  harmonizing  wonderfully  with  the  bark  of  the 
mesquite  and  other  trees  which  it  haunts.  During  the  heat  of  the  day 
it  lies  basking  on  the  trunk  or  exposed  branches  of  trees,  and  retires 
to  holes  in  trees  or  among  the  roots  at  night.  In  the  winter  it  hiber- 
nates in  shallow  holes  in  the  ground  or  under  stones  or  other  shelters. 
During  the  cool  of  the  day  they  are  actively  in  search  of  food,  which 
consists  mainly  of  tree-inhabiting  insects.  In  the  breeding  season 
the  male  takes  on  a  steely  blue  sheen  about  the  throat  and  head.  The 
courtship  and  mating  activities  are  rather  striking.  The  male  stands 
in  front  of  the  female  with  his  brilliant  throat  inflated  and  thus  dis- 
played to  the  utmost;  then  raises  himself  up  and  down  on  the  fore  legs 
with  a  quick  rhythm.  This  the  female  seems  to  watch  as  though  fas- 
cinated and  is  soon  won.  The  nest  is  dug  in  loose  soil  in  the  form  of 
a  fairly  deep  tunnel  in  a  sloping  bank.  Excavation  of  the  nest  is  ac- 
complished with  the  hind  feet  as  in  tortoises.  The  eggs,  which  are 
much  like  tortoise  eggs  in  appearance,  number  a  dozen  or  more,  and 
when  laid  are  in  a  stage  equivalent  to  about  a  72-hour  chick. 

Draco  volans  (Fig.  138,  C),  the  flying  dragon,  is  the  best  example  of 
the  volant  type  of  lizard.  The  body  is  dorso-ventrally  depressed  and 
the  skin  is  stretched  out  into  two  fan-shaped,  folding  membranes, 
which  are  supported  on  five  or  six  of  the  greatly  elongated  ribs.  On 
the  neck  are  three  hooks  which  probably  enable  the  animal  to  secure 
a  hold  when  alighting  from  a  flight.  The  wings  are  mere  parachutes 


250 


VERTEBRATE  ZOOLOGY 


and  do  not  in  any  sense  serve  as  propellers.    Only  short  soaring  leaps 
from  limb  to  limb  or  between  adjacent  trees  can  be  accomplished. 


FIG.  139. — Group  of  Lacertilia,  II.  A,  Horned  Toad,  Phrynosoma  cornutwn; 
B,  Gila  monster,  Heloderma  horridum;  C,  Anguis  fragilis  (the  Glass  Snake  or 
Blind- Worm);  D,  Cape  Monitor,  Varanus  albigularis;  E,  Galapagos  Sea-Lizard, 
Amblyrhynchus  cristalvs;  F,  Moloch  horridus.  (All  redrawn,  A,  after  Gadow; 
others  after  Lydekker.) 

When  the  animal  is  at  rest  the  " wings"  are  folded  against  the  sides. 
The  flying  dragons  are  natives  of  Indo-Malayan  countries.    Not  much 


REPTILIA  251 

is  known  about  their  habits,  but  it  is  said  that  they  live  among  gor- 
geous flowers  whose  colors  they  closely  approximate.  Doubtless  this 
camouflage  aids  the  lizard  in  securing  insect  food. 

Phrynosoma  cornutum  (Fig.  139,  A),  the  horned  toad,  is  chosen  as 
a  desert  type.  Of  course  this  animal  is  not  a  toad  at  all  but  a  short, 
flat,  spiny  lizard,  with  a  very  reduced  tail,  a  character  that  evidently 
suggested  the  name  "  toad  "  for  it.  They  live  in  the  semi-arid  regions 
of  the  southwestern  States  and  in  Mexico.  The  only  water  they  seem 
to  take  is  in  the  form  of  dewdrops,  and  they  are  capable  of  living  for 
a  long  time  without  any  water,  growing  flatter  and  lighter  as  desicca- 
tion progresses.  Their  chief  food  appears  to  be  ants,  though  other 
small  insects  are  not  unwelcome.  They  are  fond  of  basking  in  the  hot- 
test sun  during  the  day,  but  when  night  approaches  they  bury  them- 
selves in  the  sand  while  still  warm  from  the  sun,  leaving  only  the  top 
of  the  head  and  the  horns  exposed.  The  nostrils  are  provided  with 
valves  to  prevent  the  inhalation  of  the  fine  sand.  They  are  colored  a 
dull  sandy  gray,  and  this,  together  with  their  rugose  appearance, 
makes  them  very  inconspicuous  against  the  usual  desert  background. 
One  curious  habit  which  the  writer  had  heard  of  with  considerable 
skepticism  and  only  believed  when  he  saw  it  with  his  own  eyes,  is  that 
of  squirting  a  tiny  stream  of  blood  out  of  the  eye,  when  cornered  and 
in  danger.  The  blood  is  expelled  from  the  inner  corner  of  the  eye  and 
can  be  shot  to  a  distance  of  two  feet  or  more.  What  advantage  is 
gained  by  this  curious  habit  no  one  seems  to  know.  The  horned  toad 
is  a  docile  little  creature  and  is  easily  tamed.  Of  all  animals  that  the 
writer  has  experimented  with,  they  are  the  most  readily  hypnotized 
by  turning  them  on  the  back  and  pressing  gently  but  firmly  against 
the  ventral  surface. 

Anguis  fragilis  (Fig.  139,  C),  the  slow-worm  or  blind-worm,  is  also 
called  in  some  sections  of  the  country  the  "glass  snake."  These 
lizards  are  true  fossorial  or  burrowing  types.  They  are  limbless  forms, 
representing  the  climax  of  degeneration  among  the  Lacertilia.  There 
is  a  current  legend  of  the  Southern  States  that  this  creature  can  be 
shattered  by  a  blow  into  a  number  of  pieces  and  that  these  pieces  get 
together  again  into  an  entire  animal,  which  then  goes  on  its  way  re- 
joicing. The  truth  underlying  the  legend  is  that,  like  other  lizards, 
the  tail  is  quite  brittle  and  readily  knocked  off  by  a  blow  from  a  stick. 
Both  animal  and  tail  wriggle  about  vigorously  after  such  violent  treat- 
ment, but  only  the  body  is  able  to  resume  the  journey. 


252  VERTEBRATE  ZOOLOGY 

Basiliscus  americanus  (Fig.  138,  F),  the  American  basilisk,  may 
be  chosen  as  an  amphibious  type.  It  is  a  large,  conspicuous  lizard 
about  a  yard  in  length.  It  is  characterized  by  a  very  pronounced 
dorsal  crest,  which  looks  like  a  fin,  a  secondary  sexual  character 
limited  to  the  males.  They  lie  on  the  branches  of  trees  overhanging 
the  water  and  at  the  slightest  danger  drop  off  into  the  water  and  swim 
rapidly  ashore,  using  the  fin  only  as  a  rudder. 

Amblyrhynchus  cristatus  (Fig.  139,  E),  the  sea  lizard,  is  as  near  an 
approach  to  a  true  aquatic  type  as  the  Lacertilia  afford.  These  rather 
large,  heavy-bodied  lizards  inhabit  certain  rocky  shores  on  the  Gal- 
apagos Islands.  They  are  great  swimmers,  using  the  flattened,  finned 
tail  as  a  propeller.  They  habitually  feed  upon  the  seaweeds  that 
abound  beyond  the  breakers,  and  they  have  to  weather  the  waves 
in  order  to  secure  their  food.  Often  they  prefer  the  really  dangerous 
breakers  to  their  enemies  on  land,  and  seek  shelter  in  the  sea. 

Moloch  horridus  (Fig.  139,  F)  is  one  of  the  strangest  of  lizards. 
Its  integument  is  remarkable  for  its  heavy  spines.  This  animal  has 
been  described  as  a  lizard  ant-eater  and  its  peculiarities  are  considered 
to  be  primarily  adaptations  for  the  ant-eating  life.  It  certainly  looks 
to  be  well  protected  to  withstand  the  attacks  of  ants.  One  peculiar 
feature  of  the  integument  has  attracted  considerable  attention;  for 
the  skin  is  said  to  be  hygroscopic,  capable  of  absorbing  moisture 
from  the  air.  This  strange  lizard  rivals  in  bizarre  appearance  the  most 
fanciful  monsters  of  long  ago.  Only  its  small  size  redeems  it  from 
utter  frightfulness  of  aspect. 

The  only  venomous  lizard  is  the  gila  monster  (Fig.  139,  B),  Helo- 
derma  horridum,  a  large,  heavy-bodied  lizard  of  the  arid  lands  of  our 
southwest  and  Mexico.  It  has  fang-like  recurved  teeth,  which  are  so 
grooved  as  to  form  ducts  for  the  poisonous  secretion  of  the  labial 
glands.  The  Gila  is  conspicuously  marked  with  contrasting  black 
and  orange  patches  and  is  often  cited  as  an  example  of  warning  color- 
ation, a  common  phenomenon  among  venomous  reptiles. 

The  largest  living  lizard  is  the  monitor  (Fig.  139,  D),  Varanus  sal- 
vator,  a  species  that  reaches  a  length  of  seven  feet  or  more.  Apart 
from  its  great  size  the  Monitor  is  a  very  generalized  lizard,  differing 
very  little  from  the  primitive  lizard-like  reptile,  Varanops,  which 
lived  in  Permian  times.  In  Southern  China  and  the  Malaysian 
region,  where  this  lizard  has  its  home,  it  is  hunted  by  dogs  and  used 
for  food. 


REPTILIA  253 

The  largest  American  lizard  is  Iguana  tuberculata  (Fig.  138,  D),  a 
native  of  South  and  Central  America.  The  Iguana  reaches  a  length 
of  five  or  six  feet.  Its  habits  are  much  like  those  of  the  Basilisk. 

Sub-order  3.  Chamaeleontes  (Chameleons). — The  chameleons 
are  the  most  highly  specialized  of  the  lizards.  The  body  is  laterally 
compressed,  the  tail  prehensile,  the  toes  are  parted  in  the  middle  into 
two  groups  used  for  grasping,  a  group  of  three  being  opposed  by  a 
group  of  two.  Most  of  them  are  African  or  Madagascan,  though  one 
species  (Chamceleon  vulgaris]  extends  into  Southern  Europe. 

As  an  example  of  extreme  arboreal  specialization  the  group  is  of  un- 
usual interest.  Two  characters  of  chameleons  have  become  noto- 
rious: their  ability  to  change  color  and  their  habit  of  " shooting"  in- 
sects with  their  tongues.  Accounts  of  their  color  versatility  are 
exaggerated,  but  the  fact  remains  that  they  are  probably  the  most  ef- 
fective color  changers  known,  having  a  range  from  very  light  gray 
to  leaf  green;  and  the  change  can  be  made  in  a  few  seconds.  The 
tongue  is  capable  of  "shooting"  a  fly  at  a  distance  of  seven  inches 
and  the  aim  is  unerring.  Probably  the  aim  is  improved  by  the  cu- 
riously modified  eyelids  which  are  grown  together  with  the  exception 
of  a  mere  pin-hole  in  the  center.  Apparently  the  tongue  aims  at  the 
exact  point  of  focus  of  the  two  eyes.  Several  signs  of  racial  senes- 
cence are  displayed  by  chameleons:  their  high  compressed  bodies, 
their  lack  of  scales,  and  the  specialized  eyes,  feet  and  tails. 

An  excellent  account  of  the  activities  of  the  chameleon  is  given  by 
Gadow,  accompanied  by  a  composite  illustration  (Fig.  140). 

"  It  is  most  interesting  to  watch  them  stalking  their  prey.  Sup- 
pose we  have  introduced  some  butterflies  into  their  roomy  cage, 
which  is  furnished  with  living  plants  and  plenty  of  twigs.  The 
Chameleons,  hitherto  quite  motionless,  perhaps  basking  with 
flattened  out  bodies  so  as  to  catch  as  many  of  the  sun's  rays  as 
possible,  become  at  once  lively.  One  of  them  makes  for  a  butter- 
fly which  has  settled  in  the  furthest  upper  corner  of  the  cage. 
With  unusually  fast  motions  the  Chameleon  stilts  along  and 
across  the  branches  and  all  seems  to  go  well,  until  he  discovers 
that  the  end  of  the  branch  is  still  8  inches  from  the  prey,  and  he 
knows  perfectly  well  that  7  inches  are  the  utmost  limit  to  a 
shot  with  his  tongue.  He  pauses  to  think,  perhaps  with  two  limbs 
in  the  air,  but  stability  is  secured  by  a  judicious  turn  of  the  tail. 
After  he  has  solved  the  puzzle,  he  retraces  his  steps  to  the  base 


254 


VERTEBRATE  ZOOLOGY 

of  the  branch,  climbs  up  the  main  stem,  creeps  along  the  next 
branch  above,  and  when  arrived  at  the  7  inch  distance  he  shoots 
the  butterfly  with  unerring  aim.  The  capacity  of  the  mouth 


FIG.  140. — Chamseleons.      A-D,  C.  vulgaris,  showing  various  attitudes  and 
changes  of  color;  C.  pumulis  in  upper  right  hand  corner.    (From  Gadow.) 


REPTILIA  255 

and  throat  is  astonishing.    A  full  grown  Chameleon  will  catch, 

chew,  and  swallow  the  largest  moth." 

While  we  may  object  to  the  statement  that  a  Chameleon  " pauses 
to  think"  or  "knows  perfectly  well,"  we  cannot  but  admire  the  vivid- 
ness of  the  verbal  picture  here  presented. 

DIVISION  II.  OPHIDIA  (SNAKES) 

Snakes  may  be  defined  as  Sauria  or  Squamata  that  have  the  right 
and  left  halves  of  the  lower  jaw  connected  with  an  elastic  ligament, 
which  enables  the  mouth  to  stretch  much  more  widely  than  it  other- 
wise could;  they  are  limbless  or  at  best  have  rudimentary  limbs  under 
the  skin,  as  in  the  pythons.  They  represent  a  more  advanced  stage  of 
specialization  than  do  the  lizards  and  are  a  much  more  modern  devel- 
ment  than  any  of  the  other  living  reptilian  groups.  In  certain  respects 
the  snakes  are  degenerate.  As  in  the  eel-like  fishes  and  amphibians 
the  greatly  elongated  body  is  accompanied  by  loss  of  limbs. 

The  majority  of  the  snakes  have  the  quadrate  very  loosely  articu- 
lated with  the  squamosal;  which  aids  in  increasing  the  gape  of  the  jaws 
and  enables  snakes  to  swallow  objects  greater  in  diameter  than  their 
own  bodies.  This  is  well  shown  in  the  illustration  (Fig.  141,  A)  repre- 
senting a  python  swallowing  a  large  bird. 

The  vertebral  column  consists  sometimes  of  nearly  three  hundred 
vertebrae,  which  are  little  if  at  all  specialized  in  the  different  regions 
of  the  body.  The  skin  is  covered  with  scales  devoid  of  bony  cores. 
The  ventral  scales  are  usually  broad,  band-like  and  erectile,  and  are 
used  as  an  accessory  to  locomotion;  for  they  point  backwards  and 
thus  give  a  good  friction  surface  against  the/ground  or  trunks  of  trees. 
The  outer  skin  is  shed  several  times  a  year  all  in  one  piece.  The  eyes 
have  no  lids,  but  each  eye  is  covered  with  a  watch-glass-shaped  mem- 
brane which  is  transparent  and  is  shed  when  the  rest  of  the  skin  is 
moulted.  This  explains  why  the  snake  is  blind  shortly  before  and 
during  the  moult;  for  the  dead  eye-membrane  is  opaque.  The  ear  is 
peculiar  in  that  the  columella  has  a  fibrous  pad  at  the  outer  end,  which 
plays  against  the  quadrate;  so  that  when  the  quadrate  is  pulled  away 
from  the  skull  in  swallowing,  the  columella  must  be  so  dislocated  as 
to  produce  a  tremendous  roaring  sensation,  if  the  auditory  organ  is 
at  all  sensitive.  Fortunately,  perhaps,  the  snake  has  not  a  keen  sense 
of  hearing.  The  tongue  is  very  slender  and  forked  and  is  used  as  a 


256 


VERTEBRATE  ZOOLOGY 


tactile  organ;  for  it  is  thrust  out  with  a  flickering  motion  against  any 
object  that  requires  investigation. 

The  internal  organs  are  greatly  elongated,  having  the  relations  that 
those  of  a  lizard  would  have  if  it  were  stretched  out  to  more  than  twice 
the  normal  length.  The  copulatory  organ  is  usually  covered  with  re- 


FIG.  141. — Group  of  Ophidia.  A,  African  Python  (Python  seba)  swallowing  a 
bird;  B,  Cobra,  Naja  tripudians;  C,  Crolalis  durissus  (Rattlesnake);  D,  Banded 
Sea-Snake,  Platurus  laticaudatus.  (Redrawn  after  Lydekker.) 

curved  hooks  and  spines,  which  ensures  prolonged  copulation  in  spite 
of  writhing  of  the  two  bodies.  Though  the  majority  of  snakes  lay  eggs, 
many  of  them,  including  the  common  garter  snakes,  are  viviparous. 
Accordirg  to  Gadow,  "Snakes  are  intelligent  creatures;  some  be- 
come quite  affectionate  in  captivity,  but  most  of  them  are  of  a  morose 
disposition,  and  they  do  not  care  for  company."  So  far  as  the  average 
man  is  concerned  this  feeling  is  mutual;  for  the  first  human  reflex  is 
to  kill  a  snake  on  sight.  Whether  this  is  the  result  of  tradition  or  is  a 


REPTILIA  257 

residual  instinct  dating  back  to  the  arboreal  period  of  man's  ancestry, 
we  cannot  say.  This  much  should  be  said  for  the  snakes,  however, 
that  most  of  them  deserve  nothing  but  kindly  treatment,  since  they 
are  far  more  beneficial  than  many  animals  that  have  a  much  better 
reputation.  It  would  appear  that  the  few  venomous  snakes  have 
given  a  bad  name  to  the  whole  group. 

Snake  Venom. — Many  snakes,  but  a  small  percentage  of  the 
whole  group,  are  more  or  less  venomous,  but  as  a  rule  they  are  much 
less  deadly  than  they  are  supposed  to  be.  Unfortunately  there  is 
no  simple  criterion  for  distinguishing  the  poisonous  snakes  from 
the  non-poisonous.  One  merely  has  to  acquaint  himself  with  the 
habitat  and  appearance  of  the  various  snakes  native  to  the  country 
in  which  he  resides  or  in  which  he  is  sojourning.  The  poison  is 
secreted  in  a  pair  of  enlarged  labial  glands,  homologous  with  the 
parotid  glands  of  the  mammals.  A  duct  leads  from  these  glands 
to  the  hollows  of  the  paired  tubular  fangs.  The  strike  of  the  snake 
presses  upon  the  gland  and  causes  the  poison  to  exude  from  the  tip 
of  the  fang  into  the  deepest  part  of  the  wound.  Fortunately  for 
us  there  are  only  five  kinds  of  venomous  snakes  in  the  United 
States:  coral  snakes,  water  moccasin,  copperhead,  rattle-snakes,  and 
opisthoglyphs. 

There  are  two  species  of  coral  snakes  both  belonging  to  the  genus 
Elaps;  both  are  native  to  the  Southern  States.  They  are  extremely 
conspicuous  owing  to  the  vivid  contrasting  bands  of  red,  black  and 
yellow,  another  example  of  the  so-called  warning  coloration.  Accord- 
ing to  Gadow,1  "  the  gape  of  the  mouth  is  so  limited  that  these  beau- 
tiful snakes,  although  possessing  strong  poison,  are  practically  harm- 
less to  man."  E.  R.  Dunn,2  however,  has  recently  called  attention 
to  records  of  six  deaths  from  coral  snake  bites  in  the  United  States. 

The  water  moccasin  (Ancistrodon  piscivorus),  the  so-called  "  cot- 
ton mouth,"  is  a  large,  heavy  >  aquatic  species  that  reaches  a  length 
of  five  or  six  feet.  It  is  really  a  species  of  rattle-snake  without 
a  rattle.  This  snake  has  the  reputation  of  being  by  far  the  most 
venomous  of  all  North  American  snakes. 

The  copperhead  (Ancistrodon  contortrix),  another  "  rattler  "  with- 
out a  rattle,  ranges  from  Massachusetts  to  Florida  and  west  to  Texas. 

The  true  rattle-snakes  comprise  a  number  of  species  belonging  to 

1  Cambridge  Natural  History,  Vol.  VIII,  p.  635. 

2  Science,  N.  S.,  Vol.  LXII,  No.  1605,  pp.  308-9. 


258  VERTEBRATE  ZOOLOGY 

the  genus  Crotalus  (Fig.  141,  C).  Of  these  the  Texas  rattler  is  much 
the  largest  and  that  of  Canada  the  smallest.  The  largest  known  spec- 
imens reach  a  length  of  seven  feet  and  are  stockily  proportioned.  The 
bite  is  serious  but  seldom  fatal.  The  rattle  of  the  " rattler"  is  a  curi- 
ous structure,  made  by  leaving  the  end  of  the  moulted  skin  attached 
to  the  tip  of  the  tail,  each  moult  adding  a  new  ring  to  the  rattle.  The 
rattling  sound,  which  is  more  like  a  shrill  hiss,  is  made  by  quivering  the 
tail,  a  movement  of  excitement  or  fear  rather  than  a  purposeful  warn- 
ing signal.  Nevertheless  it  is  a  sound  that,  even  when  heard  for  the 
first  time,  causes  one  to  "bring  up  all  standing"  and  watch  one's  step. 
Give  a  rattle-snake  half  a  chance  and  he  will  run  away  without  at- 
tempting to  attack. 

ADAPTIVE  RADIATION  AMONG  THE  SNAKES 

Although  somewhat  limited  in  their  adaptive  versatility  by  the 
lack  of  limbs,  the  snakes  show  quite  a  wide  range  of  specialization 
for  the  various  life  zones.  The  more  generalized  types  are  the  common 
ground  snakes  that  have  holes  in  the  ground  merely  as  retreats  in 
time  of  danger  or  for  hibernation. 

A  great  many  arboreal  types  have  been  developed,  as  the  structure 
of  the  snake  is  peculiarly  well  adapted  for  that  type  of  climbing,  for 
which  we  have  no  other  name  than  "serpentine."  The  Boidce  (boa- 
constrictors)  are  typical  examples  of  arboreal  snakes  (Fig.  141,  A). 
These  large,  rapacious  creatures  secure  their  prey  by  dropping  upon 
it  out  of  trees  and  crush  it  to  death  within  their  powerful  coils.  The 
largest  of  these  snakes  are  upwards  of  twenty  feet  long,  about  six 
inches  in  diameter,  and  capable  of  crushing  a  tiger  or  a  stag.  They 
are  unable,  however,  to  eat  such  large  prey,  their  limit  being  rabbits 
and  fairly  large  birds,  which  they  are  able  to  swallow  whole  without 
difficulty.  There  are  several  types  that  are  more  highly  specialized 
for  arboreal  life  than  the  Boidse.  Among  these  are  the  members  of 
the  family  Colubrince,  which  are  characterized  by  their  great  length 
and  slenderness,  and  by  the  great  flexibility  of  the  prehensile  tail. 

Another  adaptive  type  is  that  which  is  native  of  the  arid  regions  and 
which  has  adopted  the  burrowing  habit  to  protect  itself  against  the 
extremes  of  temperature  so  characteristic  of  desert  regions.  A  spe- 
cialized burrowing  type  is  represented  by  the  genus  Typhleps,  of 
which  there  are  about  one  hundred  species.  They  dig  typical  burrows 
in  the  ground  in  which  they  spend  much  of  their  time. 


REPTILIA  259 

The  marine  snakes  are  fine  examples  of  a  purely  aquatic  type,  that 
never,  except  for  breeding  purposes,  come  out  of  the  water.  The 
species  Platurus  laticaudatus  (Fig.  141,  D)  illustrates  the  structural  and 
functional  adaptations  for  marine  life.  They  are  laterally  compressed 
in  the  tail  region,  with  dorsal  and  ventral  fin  folds;  their  mode  of 
swimming  is  precisely  like  that  of  eels.  They  are  decidedly  venomous. 
They  are  viviparous,  the  female  coming  ashore  to  give  birth  to  her 
young  among  the  rocks.  The  new-born  young  are  about  two  feet 
long  and  much  less  specialized  for  aquatic  life  than  the  adults. 

The  Cobra  (Fig.  141,  B),  Naja  tripudians,  is  perhaps  the  king  of  all 
the  snakes,  and  with  a  description  of  its  habits  we  shall  bring  this 
brief  account  of  a  not  too  pleasant  topic  to  a  close.  The  writer  finds 
it  impossible  to  wax  even  moderately  enthusiastic  about  snakes.  The 
cobra  is  a  native  of  India,  China  and  Malaysia.  Very  large  specimens 
reach  a  length  of  six  feet ;  but  it  is  not  for  their  size  that  the  cobras  are 
so  noteworthy,  but  for  their  striking  appearance,  their  venomousness 
and  their  sacredness.  They  are  distinguished  by  the  huge  hood  or 
neck  swelling,  upon  which  appears  a  color  pattern  resembling  a  death 's 
head  or  a  pair  of  spectacles,  depending  on  the  strength  of  one's  imag- 
ination. They  are  an  almost  invariable  accompaniment  of  the 
typical  Indian  conjurer,  who  charms  them  and  makes  them  dance  to 
his  weird  music.  The  dance  is  done  by  erecting  the  head  with  inflated 
hood  and  by  waving  it  back  and  forth  to  the  rhythm  of  the  music. 
The  cobra  is  by  nature  docile  and  has  no  inclination  to  bite;  but  when 
it  does  strike  it  is  a  serious  matter,  and  the  number  of  victims  of  cobra 
bite  every  year  is  appalling.  Some  of  the  natives  possess  snake  stones, 
a  sort  of  porous  material  that  appears  to  have  the  property  of  absorb- 
ing the  poison.  The  owner  of  such  a  stone  is  deemed  by  his  acquaint- 
ances to  possess  a  priceless  talisman.  In  India  the  cobra  is  considered 
a  sacred  animal  and,  on  that  account,  no  systematic  campaign  of 
extermination  has  been  started  against  it. 

In  concluding  this  chapter  on  reptiles  it  may  be  said  that  no  ac- 
count of  development  has  been  given,  for  the  reason  that  reptilian  and 
avian  embryology  are  so  similar  that  the  account  given  for  the  bird 
at  the  end  of  the  next  chapter  will  do  duty  for  the  reptilian  type  of 
development  also. 


CHAPTER  VIII 
CLASS  V.    AVES  (BIRDS) 

The  propriety  of  giving  class  value  to  the  birds  is  open  to  serious 
question.  Fundamentally  birds  are  flying  reptiles,  highly  specialized 
for  aerial  locomotion.  The  close  affinities  of  birds  to  reptiles  was  in 
the  mind  of  Huxley  when  he  combined  the  two  divisions  under  the 
name  S&iiropsida. 

There  is  never  the  slightest  difficulty  in  distinguishing  a  bird  from 
any  other  animal.  The  presence  of  feathers  is  of  itself  a  differentiating 
character.  All  birds,  moreover,  are  bipeds  and  have  the  fore  limbs 
modified  as  wings,  which  in  some  cases  are  rudimentary,  in  others, 
secondarily  specialized  as  flippers  for  swimming  under  water.  The 
absence  of  teeth  and  their  replacement  by  the  horny  bill  is  a  nearly 
universal  avian  character.  The  tail  is  greatly  reduced  or  foreshQrtened, 
much  as  it  is  in  man.  There  are  not,  except  in  certain  domestic  races 
of  fowls,  more  than  four  toes,  of  which  one  is  the  hajlux  or  great  toe. 
Now  none  of  these  characters,  except  the  possession  of  feathers,  really 
demarks  the  birds  from  the  reptiles;  for  some  group  of  reptiles,  living 
or  extinct,  is  characterized  by  bipedality,  by  wings,  by  beak,  by  lack 
of  teeth,  by  reduced  tail,  or  by  four  toes.  Is  a  bird  then  merely  a 
reptile  with  feathers?  In  a  sense,  yes;  if  feathers  be  taken  as  an  index 
of  a  complex  of  structural  and  functional  adaptations  for  flight.  The 
bird,  therefore,  may  be  thought  of  as  essentially  a  heavier-than-air 
flying  machine,  a  monoplane  with  propeller  planes,  a  type  of  motor 
mechanism  that  man  has  failed  to  duplicate.  One  of  the  most  effect- 
ive ways  of  presenting  an  account  of  the  bird 's  characteristic  struc- 
tural and  functional  peculiarities  is  to  compare  it  in  considerable 
detail  with  an  aeroplane. 

THE  BIRD  AN  AUTOMATIC  AEROPLANE 

The  essential  features  of  a  heavier-than-air  flying  machine  are: — • 
1,  Planes  or  wings;  2,  great  and  sustained  power,  including  fuel, 
engine,  propeller;  3,  minimum  weight  consistent  with  maximum 
rigidity  of  framework;  4,  steering  and  balancing  devices,  including 

260 


AVES 


rudder,  ailerons,  stabilizers.    Let  us  consider  the  ways  in  which  the 
bird  meets  these  requirements. 

1.  Planes  or  Wings. — The  wing  of  the  bird  (Fig.  142)  is  a  complex 
of  several  structural  elements  consisting  of:  a  framework  of  bones, 


—Anatomy  of  the  pigeon.    A,  nostril;  AD,  ad-digital  primary  feather; 

uditory  meatus;  BW,  bastard  wing;  C,  oesophagus;  CA,  right  carotid 

J,  crop;  DA,  aorta;  E,  keel  of  sternum;  F,  right  auricle;  G,  right  ventricle; 

aepatic  vein;  HI,  left  bile-duct;  H2,  right  bile-duct;  /,  distal  end  of  stomach; 

. ..,  right  innominate  artery;  IV,  posterior  vena  cava;  JA,  left  innominate  artery; 

JV,  right  jugular  vein;  K,  gizzard;  L,  liver;  M,  duodenum;  MD,  mid-digital 

primary  feathers;  MP,  metacarpal  primaries;   Ml,  preaxial  metacarpal;  M2, 

middle  metacarpal;  MS,  postaxial  metaearpal;  N,  cloacal  aperture;  Nl,  preaxial 

digit;  O,  bursa  Fabricii;  01,  proximal  phalanx  of  middle  digit;  O2,  distal  phalanx 

of  middle  digit;  P,  pancreas;  PA,  right  pectoral  artery;  PD,  predigital  primary; 

PV,  portal  vein;  PI,  first  pancreatic  duct;  P2,  second  pancreatic  duct;  PS,  third 

pancreatic  duct;  O,  pygostyle;  R,  rectum;  RC,  radial  carpal  bone;  RX,  rectrices; 

Rl,  ulnar  digit;  S,  ureter;  SA,  right  sub-clavian  artery;  SV,  right  anterior  vena 

cava;   T,  rectal  diverticulum;    U,  kidney;    UC,  'ulnar  carpal  bone;  V,  pelvis; 

W,  lung;  X,  humerus;  Y,   radius;   Z,  ulna.     (From    Hegner,    after   Marshall 

and  Hurst.) 

muscles,  nerves,  blood  vessels,  and  feathers.  The  bony  framework  is 
that  of  a  modified  fore  limb  of  which  the  human  arm  is  a  good  proto- 
type. *The  humerus  is  large  ^  has  heavy  ridges  for  the  attachment 
of  the  huge  pectoral  flight  v'  a\Uature; '  The  radius  and  ulna  are 
largely  unmodified,  thougv  'ne  stonx  is  larger  than  the  radius  and  has 


262  VERTEBRATE  ZOOLOGY 

a  larger  than  usual  head  for  muscle  attachment.  The  wrist,  hand,  and 
finger  bones  are  highly  modified  both  through  loss  of  whole  bony 
units  and  by  the  fusion  of  the  remaining  bories  into  strong  complexes. 
The  thumb  or  pollex  is  reduced  to  a  small  rudiment,  the  index  finger 
is  the  largest,  the  second  finger  fairly  well  developed,  but  there  is  no 
trace  of  the  third  and  fourth  fingers.  The  phalangeal  part  of  the  fore 
limb  is  reduced  essentially  to  a  one-fingered  condition. 

Of  the  wing  muscles  those  of  the  upper  arm  are  very  large  and 
powerful,  those  of  the  lower  arm  much  reduced,  and  those  of  the  hand 
atrophied.  The  only  movements  of  the  wings  are  those  of  elevating, 
depressing,  extending  and  flexing.  The  real  flight  muscles  are  the 
chest  muscles  or  pectorals,  massive  groups  of  fine-grained  striated 
fibers,  which  are  inserted  upon  the  keel  of  the  sternum.  These  muscle 
masses,  which  are  capable  of  prolonged  exertion  without  fatigue, 
correspond  to  the  cylinders  of  the  aeroplane  motor. 

The  wing  feathers  are  the  main  factors  in  giving  large  planing  surface 
to  the  wing.  A  feather  (Fig.  143)  from  the  morphological  standpoint, 
is  no  more  nor  less  then  an  elaborately  subdivided  scale,  rolled  up 
into  a  cylinder  proximally  and  expanded  into  a  flat  vane  at  the  distal 
end'.  The  quill  is  residue  of  the  embryonic  rolled-up  stage.  The  vane 
is  composed  of  a  number  of  subdivisions  called  barbs,  each  of  which  is 
redivided  into  minute  barbules  which  are  hooked  to  the  barbules  of 
adjacent  barbs  so  as  to  give  stability  to  the  whole  vane  and  to  make 
the  feather  as  a  whole  a  coherent,  springy  plane. :  A  single  row  of  large 
flight  feathers  grows  out  from  the  back  of  the  arm  and  hand  bones, 
each  partly  overlapping  its  neighbor.  Several  rows  of  so-called  covert? 
overlie  these  Ufee~3h1hgle  rows.  The  overlapping  arrangement  of  all 
the  feathers  contributes  greatly  to  make  the  wing  a  fairly  rigid,  but 
sufficiently  flexible  plane,  which  is  better  adapted  for  the  purpose 
than  the  perfectly  rigid  planes  of  man-made  machines.  The  wing 
differs  also  from  the  plane  in  that  it  is  jointed  and  capable  of 
being  folded  away  when  not  in  use,  or  of  regulating  its  exposed  sur- 
face by  flexures. 

2.  Power. — The  secret  of  great  and  sustained  power  lies  in  the 
capacity  to  convert  chemical  energy  into  mechanical  motion  through 
rapid  and  complete  combustion  of  fuel.  In  the  aeroplane,  gasolene  is 
the  fuel,  the  electric  spark  is  the  corp^'istion  agent  and  oxygen  the 
combustor;  in  birds  carbo hydrates r  ^Sn^ Constitute  the  fuel,  the  nerve 
impulse  is  the  combustion  agent?  anc*  Nygen  the  combustor;  the 

50 


J 


AVES  263 

wing  muscles,  especially  the  pectorals,  are  in  flying  birds  extremely 
massive,  which  means  that  a  great  excess  of  energy  is  always  avail- 
able; the  nervous  system  is  highly  efficient;  and  the  supply  of  oxygen 
is  ensured  by  the  extraordinary  development  and  unique  structure  of 
the  lungs  and  air  passages,  as  well  as  by  the  adequate  blood  supply 
and  its  circulation.  The  lungs  proper  are  not  unduly  large,  but  their 
capacity  is  greatly  increased  by  the  addition  of  large  air-sacs,  that 
branch  off  from  the  lungs.  These  air-sacs  fill  all  of  the  ccelomic 
spaces  and  even  send  fine  branches  into  the  hollows  of  the  bones.  By 
this  scheme  two  functions  are  subserved:  that  of  sending  oxygen 
directly  to  many  tissues,  and  that  of  lessening  the  weight  of  the  body. 
The  lungs  moreover  differ  from  those  of  reptiles  or  mammals  in  that 
a  through  draft  of  air  is  made  possible  through  a  system  of  excurrent 
bronchi,  passages  that  carry  used  air  out  of  the  lung  alveoli  without  in- 
terfering with  the  fresh  air  that  enters  through  the  incurrent  bronchi. 
Thus  the  bird  7s  oxygen  supply  is  much  better  provided  for  than  that 
of  any  other  vertebrate,  and  in  some  respects  approximates  that  pos- 
sessed by  the  flying  insects.  Adequate  oxidation  is  further  provided 
for  by  the  large  heart  (Fig.  142)  and  by  voluminous  blood  vessels, 
both  of  which  are  proportionately  more  generous  in  their  blood- 
carrying  capacity  than  those  of  other  vertebrates. 

The  high  temperature  of  the  bird  is  another  important  element 
in  its  power  plant.  Obviously,  the  higher  the  temperature,  the  more 
rapid  the  combustion.  The  bird 's  temperature  is  considerably  higher 
than  that  of  mammals,  as  anyone  knows  who  has  felt  the  skin  of  a  live 
fowl.  In  the  best  fliers  it  runs  up  to  110°  or  112°  F.,  even  when  the 
birds  are  at  rest.  Two  elements  are  concerned  in  maintaining  the 
characteristic  avian  temperature:  a  vaso-motor  system,  similar  to 
that  of  mammals,  and  an  unusually  effective  non-conductive  coat  of 
feathers,  which  prevents  surface  loss  of  heat;  and  no  known  material 
does  this  more  effectively  than  the  feather  coat  of  a  bird,  especially 
when  the  feathers  are  arranged  as  they  are  in  nature.  With  this 
equipment  the  bird  is  able  to  endure  the  intense  cold  of  tr»o  upper 
atmospheric  strata  without  undue  loss  of  heat  and  without  thr  least 
danger  of  freezing. 

The  alimentary  system  is  also  proportionately  effective.  It 
must  be,  for  it  is  the  fuel  refinery.  Crude  power  materials  are  taken 
into  the  crop  or  storage  tank,  are  gradually  fed  into  the  grinding  mill 
(gizzard)  and  passed  into  the, stomach  proper,  and  subsequently  into 


264  VERTEBRATE   ZOOLOGY 

the  intestines,  in  such  a  condition  that  digestion,  or  the  refining  of  the 
fuel,  is  rapid  and  complete.  Much  might  be  said  of  the  efficiency  of 
the  excretory  apparatus,  but  this  may  be  assumed. 

The  mechanics  of  propulsion  is  difficult  of  explanation  because 
of  its  extreme  complexity;  but  this  much  may  be  said:  the  wing  stroke 
is  practically  like  the  arm  stroke  in  swimming.  It  must  do  two  things : 
prevent  the  body  from  falling,  and  give  a  forward  impulse.  The  stroke 
must  therefore  be  downward  and  backward;  but  a  forward  and  up- 
ward stroke,  like  the  recovery  stroke  in  swimming,  alternates  with  the 
power  stroke.  The  possibility  of  effective  and  rapid  propulsion  de- 
pends on  the  relatively  frictionless  character  of  the  recovery  stroke. 
This  is  accomplished  by  bringing  back  the  wing  edgewise  to  the  re- 
sistance of  the  air.  Many  birds  make  progress  by  planing  up  and  down 
the  air  currents  with  nearly  rigid  wings.  In  this  phase  of  flight  man 
has  equaled,  if  not  surpassed,  the  bird. 

3.  Lightness  and  Rigidity. — Many  elements  combine  to  make  the 
bird  a  model  of  mechanical  perfection  in  this  respect.  The  skeleton 
(Fig.  144)  exhibits  instances  of  the  use  of  nearly  all  of  the  recognized 
architectural  principles  designed  for  getting  the  most  strength  and 
rigidity  out  of  the  least  material.  The  T  and  I  beam  principles  are 
used  in  many  of  the  bones,  the  most  striking  example  being  the  ster- 
num, an  ideal  T  beam.  Many  of  the  bones  are  broadened  and  flat- 
tened; there  is  much  overlapping,  as  in  the  uncinate  processes  of  the 
ribs;  and  there  is  very  extensive  fusion  of  adjacent  bones,  with 
resultant  increase  of  rigidity.  The  vertebral  column,  with  the  ex- 
ception of  the  cervical  region,  is  practically  rigid,  extensive  fusions 
having  taken  place  between  the  vertebrae  themselves,  and  between  the 
latter  and  the  bones  of  the  pelvis.  The  bones  of  the  skull  are  almost 
paper-thin,  but  are  so  fused  into  a  unit  as  to  make  a  practically 
sutureless  brain-box.  A  large  number  of  bones  are  lost,  especially  in 
the  wings  and  legs,  and  those  that  remain  are  filled  with  air  instead 
of  with  bone-marrow.  Thus  the  skeleton  of  the  birds  is,  among 
vertebrates,  much  the  lightest  for  its  size,  yet  the  strongest,  as  it 
must  be  to  withstand  the  racking  strains  incident  to  flight. 

In  a  sense  the  bird  is  also  partially  a  balloon  in  that  quantities  of 
hot  air  are  carried,  not  only  in  the  extensive  air-sac  system,  but  also 
inclosed  between  the  body  and  the  feathers  and  among  the  innumer- 
able feather  interstices.  Nearly  half  of  the  contour  volume  of  a  bird 
is  air-filled, 

•    \ 


AVES  265 

4.  Steering  and  Balancing  Devices. — The  tail  and  its  feathers 
(rectrices)  is  a  rudder  which  may  be  used  as  well  for  vertical  as  for 
lateral  steering.  Elevating  the  tail  produces  an  upward  slant,  de- 
pression a  downward  turning.  Tilting  from  side  to  side  gives  lateral 
steerage.  Expanding  the  feathers  like  a  fan,  or  closing  them  together, 
increases  or  decreases  the  effectiveness  of  the  rudder.  Balancing 
devices  are  used  especially  in  soaring,  when  irregular  wind  currents 
strike  the  outspread  wings  and  tend  to  capsize  the  vessel.  To  equal- 
ize irregularities  of  air  pressure  on  the  two  wings  the  bird  may  de- 
crease the  surface  of  the  wing  by  partially  flexing  it  at  elbow  or  shoul- 
der, or  by  twisting  the  tip  of  the  wing  so  as  to  spill  off  the  excess  wind. 
Part  of  the  stabilizing  equipment  consists  of  the  flexible  ends  of  the 
feathers  which  bend  upward  and  spill  off  the  air,  much  after  the  fash- 
ion of  the  ailerons  on  an  aeroplane.  In  the  bird  no  elaborate  stabil- 
izer is  necessary,  for  each  individual  is  an  automaton,  with  an  effective 
system  of  balancing  reflexes  ever  on  the  alert. 

Any  more  extensive  discussion  of  the  flight  adaptations  of  the  bird 
would  lead  us  into  a  technical  exposition  quite  out  of  place  in  the 
present  volume.  Enough  has  been  presented  to  impress  the  reader 
with  the  fact  that  almost  all  of  the  characters  that  distinguish  a  bird 
from  a  reptile  are  fundamentally  elements  belonging  to  its  flying 
equipment.  Unless  therefore  these  characters  were  evolved  in  con- 
nection with  flight  they  are  meaningless;  for  no  other  set  of  conditions 
could  have  called  forth  this  peculiar  combination  of  characters.^'' 

BIRDS  AND  REPTILES  COMPARED 

Apart  from  its  flight  adaptations  the  bird  has  been  shown  to  be 
extraordinarily  reptile-like.  The  avjan  egg  is  essentially  like  that  of 
the  reptile,  both  in  size  and  in  envelopes.  The  developmental  history, 
though  much  more  rapid,  as  the  result  of  higher  temperatures,  is 
essentially  reptilian.  This  speeding  up  of  the  developmental  rate  has 
evidently  been  an  important  element  in  the  evolution  of  the  bird. 
Like  the  reptile  the  bird's  jaw  consists  of  several  bones  and  articulates 
with  the  quadrate.  The  skull  bones  are  not  materially  different  from 
those  of  the  reptile  ;^  while  the  vertebrae,  in  their  variable  number 
especially  in  the  cervical  region,  and  their  lack  of  epiphyses,  are  rep- 
tilian. The  hind  limbs  and  pelvic  girdle  are  strikingly  like  those  of 
some  of  the  dinosaurs.  The  circulatory  system  is  somewhat  different 
from  that  of  any  living  reptile,  but  is  the  logical  development  of  tend- 


\ 


266 


VERTEBRATE  ZOOLOGY 


TV/I 


encies  observable  in  the  latter.  The  only  aortic  arch  is  the  right 
(but  there  is  a  reduction  of  the  left  arch  in  many  reptiles) ;  the  heart 
is  completely  four  chambered  (but  the  crocodile  heart  is  nearly  so); 
the  red  blood  corpuscles  are  nucleated  as  in  the  reptile. 

FORMAL  LIST  OF  CHARACTERS  OF  A  TYPICAL  BIRD 

External  Characters. — Body  short  and    spindle-shaped;    head, 
neck  and  trunk  clearly  denned;  tail  short  and  broad;  horny  beak  with 

patch  of  swollen  skin 
at  base,  called  cere; 
two  slit-like  oblique 
nostrils  between  beak 
and  cere;  eye  with 
upper  and  lower  lids 
and  a  complete  third 
eyelid  or  nictitating 
membrane;  auditory 
aperture  behind  the 
eye,  without  external 
ear,  leading  to  the 
tympanum;  wings  al- 
ready described;  legs 
covered  with  scales 
and  armed  with  claws. 
Feathers  (Fig.  143.) 
— A  feather  is  a  mod- 
ified scale,  that  arises 
from  a  dermal  papilla 
and  is  at  first  cov- 
ered with  an  epidermal 
sheath.  A  typical 
feather  consists  of  a 


FIG.  143. — Feathers  of  pigeon.  A ,  part  of  a  tail 
feather;  B,  filoplume;  C,  nestling  down,  cal,  cala- 
mus; inf.  umb,  inferior  umbilicus;  rch,  rachis;  sup. 
umb,  superior  umbilicus.  (From  Parker  and  Has- 
well.) 

stiff  axial  rod  or  stem, 

of  which  the  basal  portion  is  hollow  and  forms  tne  qufflor  calamus; 
the  distal  part  is  filled  with  pith  and  is  called  the  rachis.  The  rachis 
supports  the  vane  or  flat  part  of  the  feather,  which  is  composed  of 
parallel  barbs,  each  barb  divided  ii^to  numerous  barbules  along  either 
side,  that  hoolf  themselves  to  barbules  of  adjacent  barbs  and  thus 
help  to  make  a  coherent  plane  out  of  a  series  of  separate  parts.  Three 


AVES  267 

principal  types  of  feathers  are  to  be  distinguished  :  a,  contour  feathers, 
which  include  the  flight  feathers;  JD,  down  feather  s±  possessing  a  soft 
shaft  and  a  vane  without  barbules;  c,  filoplumes,  with  slender,  hair- 
like  shaft  and  few  or  no  barbs.  Feathers  are  arranged  in  tracts, 
called  pterylce,  with  naked  spaces  between,  called  apteria.  Moulting 
of  feathers  occurs  periodically,  old  feathers  being  dropped  and  new 
ones,  sometimes  of  different  color,  growing  out  of  the  old  follicles. 

The  Skin  is  dry  and  practically  without  glands.  The  only  skin 
gland  is  a  single  oil  gland  on  the  tail;  even  this  is  absent  in  some 
species. 

The  Skeleton  (Fig.  144).  —  Most  of  the  skeletal  peculiarities  have 
been  already  discussed.  The  sternum  is  keeled  except  ostriches,  etc.  ; 
ribs  have  uncinate  processes,  except  Screamers;  skull  is  rounded,  has 
large  orbits  and  the  facial  bones  are  extended  out  upon  the  beak; 
quadrate  is  movable  and  articulates  with  the  squamosal;  a  single  oc-  1 
condyle:  no  teeth,  except  extinct  forms;  cervical  vertebrae 


have  saddle-shaped  articular  surfaces,  giving  the  n§ck  great  flexi- 
bility and  rendering  the  beak  an  unusually  versatile  implement;  trunk 
vertebrae  mostly  fused;  three  or  four  free  caudal  vertebras  with  term- 
inal pygostyle:  two  cervical  and  three  to  nine  thoracic  ribs,  the  latter 
attached  to  the  sternum;  pectoral  girdle  consists  of  paired  blade-like 
scapulae,  paired  coracoids,  that  are  united  to  the  sternum,  and  free 
clavicles,  fused  in  the  middle  to  make  the  "wish-bone";  the  pelvic 
girdle  is  a  solid  bone,  consisting  of  the  fused  ischia,  ilia,  and  pubes, 
and  the  pelvis  is  firmly  fused  with  the  sacral  vertebrae;  the  wing  skel- 
eton has  been  sufficiently  described;  the  leg  skeleton  consists  of  a 
large  femur,  a  slender  fibula  and  the  long,  stout  tibio-tarsus,  composed 
of  the  fused  tibia  and  proximal  tarsal  bones;  the  ankle  joint  is  be- 
tween the  tibio-tarsus  and  the  tarso-metatarsus;  foot  has  four  digits, 
with  hallux  usually  directed  backward. 

Digestive  System.  —  Mouth  hard  and  narrow;  tongue  hard  and 
often  of  great  functional  value;  oesophagus  with  enlargement,  called 
the  crop;  stomach  with  proventriculus  that  secretes  gastric  juice,  and 
a  muscular  gizzard  or  gastric  mill;  intestine  U-shaped,  composed  of 
duodenum,  ileum  and  rectum;  between  ileum  and  rectum  are  two  cceca; 
rectum  opens  into  a  cloaca.  There  are  two  bile  ducts  but  no  gall- 
bladder; a  pancreas  empties  into  the  duodenum. 

Circulatory  System.  —  The  heart  is  large  and  four  chambered;  right 
auricle  receives  venous  blood,  left  receives  blood  from  the  lungs;  the 


FIG.  144. — Skeleton  of  a  Bird  (Common  Fowl.)  1,  prem axilla;  2,  nasal;  3, 
lachrymal;  4,  frontal;  5,  mandible;  6,  lower  temporal  arcade  in  region  of  quad- 
ratojugal;  7,  tympanic  cavity;  8,  cervical  vertebrae;  9,  ulna;  10,  humerus;  .11, 
radius;  12,  carpometacarpus;  13,  first  phalanx  of  second  digit;  14,  third  digit; 
15,  second  digit;  16,  ilium;  17,  ilio-ischiatic  foramen;  18,  pygostyle;  19,  femur; 
20,  tibio-tarsus;  21,  fibula;  22,  patella;  23,  tarso-metatarsus;  24,  first  toe;  25, 
second  toe;  26,  third  toe;  27,  fourth  toe;  28,  spur;  29,  pubis;  30,  ischium;  81, 
clavicle;  32,  coracoid;  33,  keel  of  sternum;  34,  xiphoid.  The  forked  bone  in  front 
of  7  is  the  quadrate.  (From  Shipley  and  McBride.) 
268 


AVES 


269 


•s, 

ltd 


scl.a 


iPti 

W&Z& 

2r  c 


270 


VERTEBRATE  ZOOLOGY 


right  aortic  arch  carries  all  of  the  arterial  blood  to  the  system.  Fur- 
ther details  of  the  circulatory  system  are  best  understood  from  the  il- 
lustration (Fig.  145). 

Respiratory  System. — Large  lungs  each  with  nine  thin-walled 
air-sacs.  Air  enters  bronchi,  passes  to  air-sacs  and  thence  in  a  warmed 
condition  into  the  alveoli  of  the  lungs  and  makes  its  exit  through  the 


elf 


o.t 


FIG.  146. — Brain  of  Bird  (Pigeon).  A,  dorsal;  V,  ventral;  C,  left  lateral  view; 
cb,  cerebellum;  /,  flocculus;  inf.  inf undibulum ;  m.  o,  medulla  oblongata;  o.  I, 
optic  lobes;  o.  t,  optic  tracts;  pn,  pineal  body;  II-XII,  cerebral  or  cranial  nerves; 
sp.  I,  first  spinal  nerve.  (After  Parker.) 

excurrent  bronchi.  A  complete  change  of  air  occurs  at  each  inspira- 
tion and  expiration.  The  trachea  and  the  larger  bronchi  are  kept 
open  by  means  of  rings  of  cartilage;  the  trachea  is  enlarged,  just  be- 
fore it  divides,  into  a  syrinx  or  voice  box,  a  structure  limited  to  birds 
and  that  is  in  no  way  homologous  with  the  larynx  of  mammals;  the 
mechanics  of  voice  production  in  the  bird  depends  upon  forcing  air 
through  a  flexible  valve,  which  is  set  in  vibration. 


AVES  271 

Excretory  System. — Paired  tri-lobed  kidneys  empty  by  means 
of  ureters  directly  into  the  cloaca.  Faeces  and  urine  are  given  off 
mixed.  The  bird  kidney  is  a  metanephros. 

Reproductive  System. — Testes  are  oval  and  are  situated  dorsally 
along  the  back.  Each  testis  has  a  vas  deferens  leading  to  a  seminal 
vesicle,  where  sperm  is  stored.  In  copulation  sperm  is  simply  trans- 
ferred from  the  cloaca  of  the  male  to  that  of  the  female;  for  there  is 
no  penis  in  most  birds.  The  left_pwry  op]y  is  functional,  the  right  be- 
coming vestigial  early  in  development.  The  single  oviduct  is  large  and 
complex,  and  is  provided  with  albuminous  and  shell  glands.  The  egg, 
which  is  what  is  popularly  called  the  yolk,  is  fertilized  before  it  de- 
scends very  far  into  the  oviduct,  soon  becomes  wrapped  round  with 
layers  of  albumen  and,  before  it  is  laid,  is  covered  by  a  shell  which  is 
secreted  by  glands  in  the  lower  part  of  the  oviduct.  Eggs  are  incu- 
bated outside  of  the  body  usually  by  means  of  the  body  heat  of  one 
or  both  parents. 

Nervous  System  (Fig.  146). — The  brain  is  very  short  and  broad. 
The  cerebrum  is  large  but  not  convoluted;  the  cerebellum  is  very  large 
and  complex;  optic  lobes  are  well  developed;  olfactory  lobes,  rudi- 
mentary, indicating  poor  sense  of  smell. 

Sense  Organs. — The  olfactory  epithelium  is  poorly  developed; 
sense  of  taste  is  almost  as  poorly  developed  as  the  olfactory.  The  in- 
ner ear,  especially  the  cochlea,  is  more  complex  than  in  reptiles.  The 
eye  is  the  bird's  main  dependence;  it  is  large  and  highly  organized, 
probably  keener  than  that  of  any  other  animal.  Sclerotic  plates  cover; 
the  eye-ball.  A  fan-shaped  pecten  (absent  in  Apteryges)  of  unkjKfwn 
function  is  suspended  in  the  vitreous  humor.  y^ 

THE  ORIGIN  OF  BIRDS     ^ 

About  the  origin  of  birds  palaeontology  says  but  little.  Only  one 
link  definitely  connecting  the  true  birds  with  their  reptilian  ancestry 
has  been  discovered.  This  one  link  is  the  bird-reptile  Archceopteryx, 
a  form  distinctly  intermediate  between  the  bird  and  the  reptile,  about 
which  we  shall  have  more  to  say  in  other  connections.  The  evolu- 
tion of  modern  avian  characters  from  those  seen  in  Archceopteryx  is 
easy  to  imagine,  for  all  of  the  avian  characters  are  foreshadowed  in 
this  creature,  which  though  more  reptile-like  than  any  other  bird,  is 
really  not  a  reptile  but  a  bird.  What  we  need  to  find  is  a  pro-avian 
ancestor  of  thev!>irds,  some  true  reptile  that  exhibits  unquestioned 

\ 


272  VERTEBRATE  ZOOLOGY 

tendencies  in  the  direction  of  avian  traits.  The  pterosaurs  might  seem 
at  first  thought  to  be  the  ideal  group  from  which  to  derive  the  birds, 
but  unfortunately  these  highly  specialized  flying  reptiles  are  anatom- 
ically too  different  from  birds  to  offer  any  hope  of  using  them  as  a 
connection  between  the  birds  and  the  reptiles.  The  pterosaurs  have 
arrived  at  their  flying  mechanism  in  an  entirely  different  fashion. 

The  bipedal  dinosaurs  have  been  chosen  by  some  authorities  as  the 
group  offering  the  strongest  suggestion  of  avian  affinities.  It  is  argued 
that  the  birds  took  their  origin  from  some  rather  generalized  offshoot 
of  the  Triassic  bipedal  dinosaurs,  which  developed  flight,  and,  after 
a  long  period  of  transition,  gave  rise  to  Archceopteryx  and  other 
primitive  birds.  This  is  possibly  the  best  clue  as  to  the  pro-avian 
ancestry  of  birds,  but  this  is  at  best  far  from  a  satisfying  phytogeny. 
In  lieu  of  a  definite  ancestral  group  of  reptiles  from  which  to  derive 
the  birds  the  problem  has  become  somewhat  less  concrete  and  concerns 
itself  with  an  attempt  to  explain  the  origin  of  the  flying  habit.  Three 
distinct  theories  of  the  origin  of  flight  are  held  at  the  present  time: 
that  of  the  cursorial,  that  of  the  arboreal,  and  that  of  the  diving 
origin  of  flight. 

The  theory  of  the  cursorial  origin  of  flight  was  advanced  by 
Nopcsa,  a  Hungarian  palaeontologist.  This  author  considers  that  there 


FIG.  147. — Restoration  of  a  hypothetical  pro-avis,  supposed  cursorial  ancestor 
of  birds.    (From  Lull,  after  Nopcsa.) 

is  a  very  fundamental  distinction  between  flight  based  on  membranous 
planes,  like  those  in  bats  and  pterodactyls,  and  planes  made  up  of 
feathers;  for  the  former  involves  marked  adaptations  of  the  hind 
limbs,  whereas  the  latter  involves  only  the  fore  li^bs  and  leaves 
the  hind  limbs  unchanged.  The  hind  limbs  of  birds  are  essentially 


AVES  273 

homologous  with  those  of  the  cursorial  dinosaurs.  It  is  therefore 
argued  that  the  origin  of  flight  involved  changes  in  the  fore  limbs  only 
and  that  the  beginnings  of  flight  occurred  while  running  efficiency 
was  at  its  height.  The  conclusion  is  that  the  first  birds  arose  from 
some  long-tailed  reptile  (Fig.  147)  that  sped  over  the  earth  on  its 
strong  hind  legs  and  stretched  out  its  fore  limbs  for  the  sake  of  main- 
taining balance  and  probably  flapped  these  limbs  to  aid  the  speed  of 
flight.  These  flapping  fore  limbs,  or  pro-wings;  developed  more  sur- 
face, partly  by  flattening  out  and  partly  by  the  backward  growth  of 
the  scales  of  the  posterior  margin.  Similar  large  scales  are  supposed 
to  have  developed  laterally  on  the  tail.  The  evolution  of  these  spe- 
cialized flight-scales  into  feathers  is  thought  to  have  been  a  mere 
matter  of  a  continued  increase  in  size  and  numbers,  accompanied  by 
regional  specialization;  for  in  reality  a  feather  is  morphologically  no 
more  nor  less  than  a  specialized  scale.  The  gradual  modification  of  the 
remaining  body  scales  into  feathers  would  be  the  logical  sequence 
of  events,  and  the  long  list  of  flight  adaptations  would  appear  as 
correlated  variations.  The  first  steps  in  flying  would  be  prolonged 
leaps,  aided  by  the  flapping  pro- wings;  then  short  soaring  flights 
would  be  made,  followed  by  longer  flights  accomplished  by  energetic 
flapping  of  the  wings  alternating  with  periods  of  soaring.  While 
rather  plausible  in  some  ways  tbe  theory  of  the  cursorial  origin  of 
flight  has  not  gained  any  generaMacceptance. 

The  theory  of  the  arboreal  origin  of  flight  has  met  with  more 
widespread  approval.  Two  phases  of  this  general  theory  have  been 
advanced :  the  pair- wing  theory,  and  the  four-wing  theory. 

The  "  pair- wing  !>  theory  is  derived  directly  from  a  study  of  the 
characters  of  Archceopteryx  (Fig.  148, 1).  The  long  clawed,  prehensile,f 
probably  climbing  wing-fingers  of  this  ancestral  bird  point  toward 
an  arboreal  habitat.  It  is  believed  to  have  been  not  a  true  flyer,  but 
merely  a  soarer  or  glider,  capable  of  only  short  passages  from  limb 
to  limb,  or  from  tree  to  tree.  The  lack  of  any  foundation  for  a  flight 
musculature  argues  against  the  possibility  that  the  creature  could  have 
taken  any  long  flights  in  which  propulsion  by  means  of  wings  would  be 
necessary. 

The  "  four-wing "  theory  of  Beebe  is  the  most  recent  theory 
dealing  with  the  origin  of  flight.  This  author  made  the  remarkable 
discovery  that  vestigial  flight  feathers  occur  on  the  thighs  of  a  number 
of  species  of  modern  birds.  Traces  of  similar  feathers  were  found  on 


274 


VERTEBRATE  ZOOLOGY 


the  thighs  of  Archceopteryx.     These  discoveries  led  to  the  conclusion 
that  the  first  flyers  had  wings  on  both  arms  and  legs  (Fig.  148,  J) 


FIG.  148. — Group  of  figures  to  illustrate  theories  of  the  origin  of  flight.  A,  re- 
construction of  skeleton  of  Archceopteryx  compared  with  that  of  a  pigeon,  B;  C,  D, 
silhouettes  of  pheasant  (left)  and  Archceopteryx  (right)  to  illustrate  two  wing 
theory  of  origin  of  flight;  E,  F,  G,  H,  four  stages  in  the  hypothetical  evolution  of 
the  two  winged  from  the  four  winged  bird.  I,  restoration  of  Archceopteryx,  after 
Heilmann.  J,  Tetrapteryx,  the  hypothetical  four-winged  ancestral  bird  of 
Beebe.  (Redrawn  after  Osborn's  "  Origin  and  Evolution  of  Life.") 

and  used  both  sets  in  parachuting  from  trees  to  the  ground  and  from 
tree  to  tree.  Later  the  wings  of  the  legs  degenerated  as  the  tail 
feathers  took  up  the  duty  of  acting  as  a  posterior  plane,  and  the  arm- 


AVES  275 

wings  increased  in  size  and  effectiveness  as  motor  organs,  as  shown  in 
Fig.  148,  E,  F,  G,  H. 

Gregory's  compromise  theory  of  the  origin  of  flight  is  perhaps 
more  nearly  acceptable  than  any  of  those  hitherto  given,  and  is  here- 
with presented  in  his  own  words: 

"The  pro-aves  were  surely  quick  runners,  both  on  the  ground  and 
in  the  trees,  but  it  is  not  clear  whether  the  upright  position  was  first 
attained  upon  the  ground  or  in  the  trees.  Thewery  early  acquired 
habit  of  perching  upright  on  the  branches,  as  shown  by  the  consoli- 
dated instep  bones,  grasping  first  digit  and  strong  claws  of  Archceop- 
teryx.  Their  slender  arms  ended  in  three  long  fingers  provided  with 
large  claws  which  were  at  first  doubtless  used  in  climbing.  These 
active  pro-aves  contrasted  widely  in  habits  with  their  sluggish  remote 
reptilian  forebears.  In  pursuit  of  their  prey  they  jumped  lightly  from 
branch  to  branch  and  finally  from  tree  to  tree,  partly  sustained  by  the 
folds  of  skin  on  their  arms  and  legs  and  later  by  the  long  scale-feathers 
of  the  pectoral  and  pelvic  l  wings '  and  tail.  That  they  held  the  arms 
and  legs  perfectly  still  throughout  the  gliding  leap  appears  doubtful, 
for  all  recent  animals  that  do  that  have  never  attained  true  flight.  I 
cannot  avoid  the  impression  that  a  vigorous  downward  flap  of  the 
arms  even  before  they  become  efficient  wings,  would  assist  in  the 
' take-off'  for  the  leap,  and  that  another  flap  just  before  landing  would 
check  the  speed  and  assist  in  the  landing." 

Diving  Origin  of  Flight. — So  far  as  the  writer  is  aware,  no  one 
has  proposed  a  theory  of  flight  involving  the  idea  that  flight  may 
have  originated  in  connection  with  soaring  over  the  water  and  diving 
after  fish.  Yet  there  are  certain  considerations  that  strongly  support 
such  a  conception.  According  to  this  view  the  pro-aves  used  the  fore 
limbs,  together  with  their  membranes  and  elongated  scales  (possibly 
also  the  similar  structures  of  the  legs),  as  planes  to  aid  in  diving.  The 
value  of  such  accessories  is  obvious;  the  dive  being  more  definitely 
directed,  the  descent  being  made  flatter  so  as  to  carry  the  diver 
farther  out  from  shore,  and  the  force  of  the  plunge  being  eased  up 
sufficiently  to  avoid  shock.  If  the  wings  were  flapped  more  or  less  a 
longer  glide  out  over  the  water  could  be  made,  and  possible  circling 
movements  could  be  made  over  the  water  while  searching  for  fish. 
It  would  appear  therefore  that  the  use  of  the  pro-wings  as  planes  in 
diving  would  serve  as  useful  a  function  as  in  running  or  leaping  from 
bough  to  bough. 


276  VERTEBRATE  ZOOLOGY 

We  would  then  have  to  suppose  that  some  of  the  archaic  diving 
birds,  such  as  the  penguins,  underwent  a  specialization  of  the  prim- 
itive wings,  using  them  for  under-water  " flying";  that  others,  such  as 
the  grebes,  never  developed  them  into  fully  effective  organs  of  flight; 
while  still  others,  such  as  the  loons,  became  good  flyers  though  still 
retaining  their  diving  propensities.  According  to  Dr.  Coues,  the 
loon  practically  flies  under  the  water,  using  the  wings  as  well  as  the 
feet  as  propellers.  The  strong  flying  sea-birds  would  then  be  derived 
from  ancestral  diving  types  that  had  gradually  perfected  their  flight; 
while  land-birds  of  all  sorts  would  be  derivatives  of  the  sea-birds. 
There  are,  in  fact,  many  evidences  that  the  sea  is  the  ancestral  home 
of  the  birds  and  that  they  have  invaded  the  land  in  comparatively 
recent  times.  If  one  turns  to  page  289,  where  the  orders  of  carinate 
birds  are  listed,  he  will  note  that  Brigade  I  (largely  archaic  birds) 
consist  almost  entirely  of  water  birds,  while  Brigade  II  (largely 
modern  types)  consists  exclusively  of  land  birds,  with  the  arboreal 
birds  confined  to  the  more  highly  specialized  sub-orders.  If  this 
classification  represents  an  approximation  to  the  phylogenetic  order, 
the  arboreal  birds,  instead  of  being  the  most  primitive  (as  the  theory 
of  arboreal  origin  of  flight  maintains),  are  a  modern  product,  and  life 
in  the  trees  is  a  modern  habit. 

Archceopteryx,  of  course,  seems  to  militate  against  the  diving 
origin  of  flight,  for  it  is  assumed  to  be  a  climbing  arboreal  bird.  But 
might  not  climbing  be  equally  appropriate  as  an  aid  in  scaling  cliffs 
after  diving  and  swimming  in  the  water?  Moreover,  the  teeth  of 
Archxopteryx  would  be  of  great  service  in  seizing  fish.  On  the  whole, 
then,  the  existence  of  Archceopteryx  is  no  more  a  barrier  to  the  ac- 
ceptance of  the  diving  than  to  the  cursorial  origin  of  flight;  while 
other  considerations  appear  to  make  the  former  more  probable  than 
the  latter. 

ARCHAIC  BIRDS  (ARCH^ORNITHES) 

It  is  customary  to  divide  all  birds  into  two  sub-classes:  Archceor- 
nithes,  consisting  of  but  one  species  (Archceopteryx  liihographica) ; 
and  Neornithes,  including  all  other  birds  living  and  extinct. 

It  is  highly  probable  that  the  period  of  avian  evolution  began  not 
later  than  the  Triassic;  hence  the  birds  are  the  latest  of  the  verte- 
brates to  have  made  their  appearance  in  the  world.  The  earliest 
actual  birds  whose  fossil  remains  have  been  found  are  not  materially 


AVES 


277 


different  from  modern  birds, 
fully  a  bird  is  Archceopteryx. 


The  only  avian  creature  that  is  not 


ARCHCEOPTERYX    (THE  LIZARD-TAILED  BIRD) 

Much  has  already  been  said  about  this  reptile-like  bird,  two  speci- 
mens of  which  have  been  recovered  from  the  Upper  Jurassic  slates  of 
Bavaria  (Fig.  149).  It 
was  a  creature  about 
the  size  of  a  crow,  but 
with  smaller  wings. 
It  had  a  number  of 
reptilian  characters  the 
-most  important  of 
which  are: — 

1.  There    is    no    true 

bill,  but  the  rep- 
tile-like jaws  were 
armed  with  coni- 
cal teeth  in  dis- 
tinct sockets  (Fig. 
150). 

2.  The  hand-wing  had 

three  fingers,  long 
and  probably  pre- 
hensile, armed 
with  curved  claws. 
These  were  proba- 
bly used  both  for 
seizing  prey  and 
for  clinging  to 
trunks  and  limbs 
of  trees. 

3.  The  sternum  had  no 

keel,  or  a  very 
poorly  developed 
one  at  best;  hence 
the  bird  could  not 
have  had  strong 


flying  muscles. 


FIG.  149. — Archceopteryx  Uthographica.  Berlin 
specimen,  c,  carpal,  d,  furcula;  h,  humerus;  r,  ra- 
dius; sc,  scapula;  u,  ulna;  I-IV,  digits.  (From  Parker 
and  Has  well.) 


278  VERTEBRATE  ZOOLOGY 

4.  The  centra  of  the  neck  and  back  vertebrae  were  biconcave,  as 

in  many  primitive  reptiles. 

5.  The  fibula  and  tibia  were  not  coalesced. 

6.  The  tail  was  long  and  lizard-like,  composed  of  about  21  free 

post-sacral  vertebrae  (Fig.  148,  A).    At  least  the  first  12  ver- 
tebrae bore  paired  flight  feathers  with  well-defined  shafts. 

7.  There  were  structures  that  have  been  interpreted  as  abdominal 

ribs. 

Two  additional  characters  that  are  not  reptilian  nor  fully  avian 
should  be  mentioned. 


FIG.  150. — Skull  of  ArchcBopteryx,  showing  teeth  and  sclerotic  plates.     (From 
Headley,  after  Dames.) 

1.  The  leg  was  rather  weak  as  compared  with  that  of  most  modern 

birds,  a  fact  that  militates  against  the  theory  of  the  cursorial 
origin  of  flight.  The  pelvic  girdle  is  much  smaller  than  in 
birds  of  the  present,  and  is  not  fused  with  the  sacral 
vertebrae. 

2.  The  feathers  of  the  wing  were  entirely  typical  both  in  form  and 

in  arrangement,  but  were  rather  small  for  the  size  of  the  body. 
The  general  contour  feathers  were  evidently  less  abundant 
than  in  a  modern  full-fledged  bird.  A  theoretical  reconstruc- 
tion of  the  plumage  is  shown  in  Fig.  148,  I. 

MODERN  BIRDS  (NEORNITHES) 

On  account  of  the  fact  that  Archceopteryx  aiffers  fundamentally 
from  all  other  birds  both  living  and  extinct,  it ,  ,  placed  in  a  separate 


AVES 


279 


sub-class:  Archceornithes.    All  other  birds  are  placed  in  the  sub-class 
Neornithes,  of  which  three  divisions  are  distinguished: 

Division  1.     Neornithes  Odontolcce  (toothed  diving  birds),  repre- 
sented by  Hesperornis  and  Baptornis. 


FIG.  151. — Hesperornis  regalis.  Restored  skeleton.  (From  Parker  and  Haswell, 
after  Marsh.) 

Division  2.  Neornithes  Ratitce  (running  birds),  exemplified  by  the 
ostrich,  and  represented  by  both  extinct  and  living  species. 

Division  3.  Neornithes  Carinatce  (keeled  or  flying  birds),  modern 
birds  mainly,  with  a  few  extinct  species. 


280 


VERTEBRATE  ZOOLOGY 


THE  TOOTHED  DIVING  BIRDS  (NEORNITHES  ODONTOLC.E) 

The  oldest  avian  remains  next  to  those  of  Archceopteryx  belong 
to  Cretaceous  times,  and  occur  in  strata  that  are  characteristically 


FIG.  152. — The  most  important  forms  of  birds'  feet,  a,  clinging  foot  of  a  swift, 
Cypselus;  b,  climbing  foot  of  woodpecker,  Picus;  c,  scratching  foot  of  pheasant, 
Phasianus;  d,  perching  foot  of  ouzel,  Turdus;  e,  foot  of  kingfisher,  Alcedo;  f,  seizing 
foot  of  falcon,  Falco;  g,  wading  foot  of  stork,  Mycteria;  h,  running  foot  of  ostrich, 
Struthio;  i,  swimming  foot  of  duck,  Mergus;  k,  wading  foot  of  avocet,  Recurvirostra; 
I,  diving  foot  of  grebe,  Podicipes;  m,  wading  foot  of  coot,  Fulica;  n,  swimming 


foot  of  tropic-bird,  Phaeton. 
f,  n,  from  regne  animal.) 


(From  Hegner,  after  Sedgwick's  Zoology:  b,  c,  d, 


marine;  for  the  other  fossils  in  these  strata  are  essentially  sea  types. 
The  bird  fossils  referred  to  evidently  belonged  to  a  type  that  was 
primarily  a  sea  diver,  as  is  evidenced  by  the  rudimentary  wings  and 


AVES  281 

flat  sternum,  and  by  the  fact  that  the  well-developed  legs  were  set  far 
back  as  in  modern  penguins.  This  species,  Hesperornis  (Fig.  151), 
was  a  large  bird  about  four  feet  in  length.  It  had  a  large  head  and  its 
jaws  were  provided  with  true  teeth  imbedded  in  sockets  of  the  max- 
illary and  dentary  bones.  In  general  appearance  it  must  have  re- 
sembled the  modern  loons  except  for  the  wings  which  were  very  much 
reduced,  consisting  merely  of  a  long,  slender  humerus,  without  any 
fore-arm  or  hand. 

Fragmentary  remains  of  another  toothed  diving  bird,  Baptornis, 
have  also  been  referred  to  this  division. 

BIRDS  OF  TO-DAY 

The  Present  Status  of  Birds.— The  birds  of  to-day  rank  with 
the  teleost  fishes  as  a  climax  group.  They  appear  to  be  at  the  height 
of  their  evolution  and  have  undergone  a  very  elaborate  adaptive 
radiation,  being  specialized  for  life  in  the  trees,  for  life  on  and  under 
the  ground  (in  caves),  for  life  in  waters  shallow  and  deep,  and  for 
life  in  the  air.  They  have  many  specialized  types  of  diet:  carnivorous, 
insectivorous,  herbivorous,  and  graminivorous. 

Modern  birds  show  also  many  signs  of  racial  senescence,  especially 
in  their  extreme  specializations  of  beaks  and  of  feet,  in  their  over- 
elaborate  integumentary  structures,  and  in  their  riotous  coloration. 
The  various  types  of  beaks  and  feet  are  well  shown  in  Figs.  152  and 
153,  and  explained  in  the  legends. 

Birds  exhibit  very  pronounced  sex-dimorphism,  the  males  usually 
being  more  highly  colored  and  with  more  elaborate  plumage,  wattles, 
spurs  and  other  excrescences;  while  the  females  are  usually  colored 
more  like  the  background  and  are  in  other  ways  much  less  specialized. 
Birds  differ  from  all  other  vertebrates  in  that  the  female  is  the 
heterozygous  sex,  yielding  two  kinds  of  eggs,  male-producing  and 
female-producing;  whereas  in  other  vertebrates  it  is  the  male  that  is 
heterozygous  and  produces  male  and  female  sperms. 

THE  RUNNING  BIRDS  (NEORNITHES  RATIT^E) 

This  division  of  modern  birds  is  comparatively  small  in  number  of 
species;  but  they  make  up  for  their  small  numbers  by  their  large  size. 
They  are  characterized  by:  absence  of  keel  to  the  sternum;  greatly 
reduced  wings  incapable  of  flight;  coracoid  and  scapula  fused  together; 


282 


VERTEBRATE   ZOOLOGY 


horny  sheath  of  the  bill  in  several  separate  pieces;  large  penis;  tail 
functionless;  no  oil  gland  on  the  tail. 

If  feathers,  wings,  air-sacs,  hollow  bones,  high  temperature,  etc., 
are,  as  we  have  maintained,  adaptations  for  flight,  we  have  no  alter- 


FIG.  153. — The  most  important  forms  of  birds'  beaks,  a,  flamingo,  Phoenicop- 
tervs;  b,  spoonbill,  Platalea;  c,  yellow  bunting,  Emberiza;  d,  thrush,  Turdus^e, 
falcon,  Falco;  f,  duck,  Mergus;  g,  pelican,  Pelicanus;  h,  avocet,  Recurvii ostra;  i, 
black  skimmer,  Rhynchops;  k,  pigeon,  Columba;  I,  shoebill,  Baloeniceps;  m,  stork, 
Anastomus;  n,  aracari,  Pteroglossus;  o,  stork,  Mycteria;  p,  bird  of  paradise,  Fal- 
dnellus;  q,  swift,  Cypselus.  (From  Hegner,  after  Sedgwick's  Zoology:  a,  b,  c,  d, 
k,  after  Naumann;  g,  i,  m,  o,  after  regne  animal;  1,  after  Brehm.) 

native  but  to  conclude  that  these  and  other  flightless  birds  have  been 
derived  by  reverse  adaptation  from  ancestors  that  were  able  to  fly. 
Possibly,  however,  the  Ratitse  represent  several  independent  off- 
shoots from  a  primitive  avian  stock,  in  which  the  powers  of  flight  had 
not  fully  developed,  and  in  which  wings  ceased  to  evolve  when  the 


AVES  283 

powers  of  running  served  a  more  important  function.  The  Ratitse 
are  a  very  old  group,  comparatively  speaking,  for  their  fossil  remains 
have  been  found  in  Cretaceous  rocks.  Six  families  of  Ratitse  are  dis- 
tinguished, four  living  and  two  extinct. 

The  Ostriches  or  Camel-birds  (Struthioniformes). — These  largest 
of  living  birds  are  more  highly  specialized  as  runners  than  are  any 
others.  The  foot  is  a  hoof-like  running  appendage  with  only  two 
toes,  with  heavy  claws  on  the  short  stout  toes.  Beneath,  the  foot 
is  heavily  padded  with  calluses.  The  beak  is  short  and  broad  but 
is  split  back  far  enough  to  give  a  wide  gape  to  the  mouth.  The 
head  is  comparatively  small;  the  neck  is  very  long  and  flexible.  The 
plumes  of  commerce  are  homologous  with  the  flight  and  steering 
feathers  of  the  flying  birds,  but  the  barbs  are  not  attached  to  one 
another  as  in  the  flat  vane  of  the  typical  feather. 

There  is  some  difference  of  opinion  as  to  how  many  species  of 
ostriches  exist.  Some  authorities  recognize  only  one  species,  Struthio 
camelus  (Fig.  154,  D) ;  others  distinguish  two  additional  species  which 
they  call  S.  australis,  and  S.  molybdophanes.  It  seems  advisable  to 
treat  these  doubtful  "species"  as  varieties  and  to  deal  with  only  one 
species  of  ostrich. 

The  ostrich  lives  in  arid  or  desert  country,  thriving  in  the  Sahara 
Desert  and  similar  environment  complexes.  It  is  able  to  make  good 
progress  in  the  sand,  for  its  foot  is  very  much  like  that  of  the  camel. 
On  hard  soil  it  is  probably  the  swiftest  runner  known,  being  able  to 
outdistance  a  good  horse  easily.  It  has,  however,  the  unfortunate 
habit  of  running  in  a  circle,  and  thus  may  be  caught  by  men  on  horse- 
back who  know  how  to  short-cut  across  the  circle  and  thus  to  inter- 
cept it.  Its  stride  is  said  to  be  fully  twenty-five  feet  in  length  and 
when  at  full  speed  the  wings  are  stretched  out  as  balancers  and 
probably  partially  lift  the  weight  off  the  ground  after  the  manner 
of  the  hypothetical  pro-avian  cursorial  ancestor  of  the  birds. 

A  single  cock  has  a  following  of  several  hens,  which  lay  their  eggs 
in  a  common  nest,  a  shallow  excavation  in  the  sand  or  dry  soil,  cov- 
ered up  with  debris.  The  eggs  are  not  left,  as  is  popularly  supposed, 
to  be  incubated  by  the  sun's  heat,  but  are  brooded  by  the  cock. 
Brooding  of  eggs  is  necessary,  for  the  eggs  would  be  chilled  and 
doubtless  killed  by  the  low  nocturnal  temperatures  characteristic  of 
deserts  and  arid  regions. 

When  cornered  the  ostrich  fights  viciously,  delivering  a  sidewise 


284  VERTEBRATE  ZOOLOGY 

kick  that  would  compare  favorably  with  that  of  a  mule.  They  also 
bite  and  peck  with  the  strong  beak,  but  the  feet  are  their  main  de- 
pendence. In  captivity  they  are  quite  tractable  and  they  are 
extensively  cultivated  on  farms  for  the  sake  of  their  valuable 
plumage. 

Two  stupid  traits  are  popularly  attributed  to  the  ostrich:  first,  that 
he  hides  his  head  in  the  sand  in  order  to  conceal  himself  from  his 
enemies;  second,  that  he  eats  tin  cans,  railroad  spikes,  and  similar 
non-nutritious  articles.  The  first  is  a  slander  on  this  alert,  wary, 
and  decidedly  intelligent  creature;  for  competent  observers  report 
exactly  the  opposite  behaviour,  in  that  when  hiding  it  crouches  low 
among  the  grasses  or  underbrush  and  only  raises  the  top  of  the  head 
and  eyes  above  the  shelter.  The  second  is  only  partially  true,  and 
there  is  method  even  in  this  apparent  show  of  madness;  for  when  the 
bird  is  in  captivity  it  sometimes  is  forced  to  use  various  unusual 
articles  for  abrasive  purposes,  in  lieu  of  gravel  or  more  suitable 
gizzard-filling  material. 

The  Rheas  (Rhei formes) . — The  rheas  are  much  like  the  ostriches 
in  general  appearance  and  in  habits,  but  are  smaller  and  less  highly 
specialized  for  running.  They  have  three  toes  furnished  with  rather 
heavy,  but  typical,  claws.  The  wings  are  better  developed  and  the 
feathers  less  plume-like  than  in  the  ostrich.  The  head,  neck,  and 
thighs  are  feathered.  The  rheas  are  popularly  confused  with  the  os- 
trich; in  fact  Rhea  americana  (Fig.  154,  A)  is  called  the  "  American 
ostrich."  This  species  lives  upon  the  pampas  of  Argentine,  southern 
Brazil,  Bolivia,  and  Paraguay.  They  are  swift  runners,  with  a  habit  of 
doubling  upon  their  pursuers  and  occasionally  lying  down  in  the  long 
grass  with  only  the  head  protruding.  Often  they  lie  in  this  position 
until  almost  trodden  upon,  apparently  relying  implicitly  on  the 
efficacy  of  their  concealment.  When  running  at  full  speed  they 
materially  aid  their  progress  by  vigorously  flapping  their  wings. 
Mating  and  nesting  habits  are  almost  identical  with  those  of  the 
ostrich. 

The  Emeus  and  Cassowaries  (Casuariiformes).  These  large  birds 
are  characterized  by:  rudimentary  wings;  long,  limp,  bifurcated  con- 
tour feathers;  no  plumes;  three  toes  with  typical  claws;  legs  propor- 
tionately shorter  than  in  the  two  preceding  families. 

There  are  several  species  of  cassowaries  (Fig.  154,  C),  native  to 
Australia  and  to  several  islands  of  the  Malay  Archipelago.  They 


AVES 


285 


live  in  wooded  country,  keeping  to  the  densest  parts.  They  are  swift 
and  apparently  reckless  runners  for  they  go  at  breakneck  speed 
through  the  heavy  underbrush,  over  logs  and  other  obstacles  six 


JblG.  154. — Group  of  Ratite  Birds.  A,  Rhea,  Rhea  americana;  B,  the  Kiwi, 
Apteryx  auslralis;  C,  Cassowary,  Casuarius  uniappendiculatus;  D,  Ostrich, 
Struthio  camelus;  E,  Emeu,  Dromceus  novce-hollandice.  (Redrawn  after  Evans. ) 


286  VERTEBRATE  ZOOLOGY 

feet  or  more  high.     Rivers  are  no  obstacles,  for  they  are  excellent 
swimmers. 

The  plumage  is  much  like  long,  soft  fur  and  is  used  for  weaving 
rugs  and  ornaments.  The  head  is  quite  a  striking  object,  generally 
blue  in  color,  with  flesh-colored  wattles  and  an  orange  stripe  down  the 
middle  of  the  back  of  the  neck.  A  black  shield  or  casque  with  green 
sides  adorns  the  top  of  the  head.  This  color  description  gives  the 
suggestion  that  such  a  head  would  be  decidedly  conspicuous,  but  our 
modern  knowledge  of  camouflage  would  lead  us  to  believe  that  in  the 
dense  woods  such  a  combination  of  colors  might  be  practically  in- 
visible. A  nest  is  made  of  leaves  and  grass  and  a  few  large  green  eggs 
are  deposited  therein.  As  in  the  other  Ratitse,  the  cock  broods 
the  eggs. 

The  emeu  (Fig.  154,  E)  is  a  native  of  Australia  and  is  not  unlike  the 
cassowary  in  habits  and  habitat,  except  that  it  lives  in  woods  that 
are  less  dense.  They  are  purely  monogamous,  differing  in  this  re- 
spect from  other  ratite  birds.  The  male  incubates  the  eggs  that  are 
laid  to  the  number  of  a  dozen  or  more  in  a  hollow,  scraped  out  of  the 
surface  soil.  The  flesh  is  palatable  and  the  subcutaneous  fat  is  used 
for  oil. 

The  Kiwis  (Apterygiformes) . — The  kiwis  (Fig.  154,  B)  are  fre- 
quently called  "New  Zealand  wingless  birds."  They  are  the  smallest 
of  the  modern  ratite  birds,  unless  we  include  the  tinamous,  whose 
ratite  affinities  are  in  question.  The  beak  is  long  and  slender;  the 
neck  and  the  legs  are  comparatively  short;  the  wings  are  more  rudi- 
mentary than  those  of  any  living  bird  and  are  completely  concealed 
beneath  the  long,  hair-like  plumage;  there  are  four  toes,  but  the  hal- 
lux  is  quite  short.  Five  species  of  the  genus  Apteryx  are  distinguished. 
These  are  distributed  on  the  various  islands  of  the  New  Zealand 
group,  where  they  occupy  wooded,  hilly  country.  These  strange 
birds  live  a  nocturnal  life,  hiding  in  burrows  of  their  own  making 
during  the  day.  The  burrows  are  dug  out  by  scratching  movements 
of  their  strong  feet.  They  can  run  much  more  swiftly  than  one  would 
expect  them  to  do,  considering  the  comparatively  short  legs.  Their 
stride  measures  at  least  a  yard  long  and  involves  leaving  the  ground 
at  every  step.  When  they  are  cornered  they  strike  viciously  with  the 
feet,  raising  the  leg  as  high  as  the  breast  and  delivering  a  downward 
blow.  Their  food  consists  mainly  of  earthworms,  which  are  best 
secured  at  night.  The  bird  seizes  the  worm  with  the  long  beak  and 


AVES  287 

gently  pulls  it  out  of  its  hole,  using  a  curious  wriggling  motion.  The 
name  "kiwi"  was  suggested  by  their  loud,  whistling  note.  The  nest, 
if  such  it  may  be  called,  is  an  enlarged  chamber  at  the  end  of  the 
tunnel-like  burrow  and  is  made  by  the  female.  The  male,  however, 
with  true  ratite  chivalry,  assumes  the  main  responsibility  of  incubat- 
ing the  two  large  eggs. 

EXTINCT  RATITE 

The  Moas  (Dinornithiformes). — -When  British  explorers  first  oc- 
cupied New  Zealand  nearly  seventy  years  ago  the  skeletons  of  gigan- 
tic wingless  birds  were  found  scattered  about  the  plains.  These 
skeletal  remains  were  in  such  a  good  state  of  preservation  that  it 
seems  probable  that  there  were  living  moas  less  than  five  hundred 
years  ago.  It  may  well  be  that  the  last  of  these  birds  were  extermin- 
ated by  the  Maoris.  Dinornis  was  in  general  appearance  not  unlike 
the  ostrich,  but  was  very  much  more  heavily  built  in  the  legs  and  had 
either  no  wing  bones  at  all  or  at  best  the  merest  rudiments  of  wings. 
The  birds  were  somewhat  taller  than  the  ostrich,  with  head  and  neck 
much  like  those  of  the  latter. 

The  Elephant  Birds  (&pyornithes) . — These  birds  probably  were 
living  in  Madagascar  less  than  two  centuries  ago.  They  are  believed 
to  have  furnished  the  factual  foundation  for  the  mythical  "Rocs"  of 
Sinbad  the  Sailor.  They  were  out  of  accord  with  these  birds  of 
oriental  fiction  in  that  they  were  incapable  of  flight  and  were  much 
less  gigantic  in  size,  being  only  about  seven  feet  in  height,  though  of 
massive  build.  The  eggs  were  surprisingly  large  in  size,  some  of  those 
which  are  still  used  by  the  natives  as  receptacles,  measuring  thirteen 
by  nine  inches  and  having  a  capacity  of  two  gallons.  This  is  the 
largest  egg  on  record,  though  doubtless  some  of  the  extinct  giant 
reptiles  had  larger  ones.  No  doubt  this  fine  bird  was  hunted  out  of 
existence  by  the  native  tribes  of  Madagascar.  Possibly  the  collecting 
of  their  eggs  was  more  destructive  to  the  species  than  was  the  slaugh- 
ter of  adults. 

APPENDIX  TO  THE  RATIT^E 

The  Tinamous  (Crypturiformes) . — The  systematic  relations  of 
this  interesting  group  of  birds  is  in  dispute,  some  authorities  placing 
them  near  the  ostriches  among  the  ratite  birds,  and  others  classing 
them  as  an  aberrant  family  of  the  order  Galliformes,  among  the  cari- 


288  VERTEBRATE  ZOOLOGY 

nate  birds.  By  considering  them  as  an  appendix  to  the  Ratitse  we 
shall  avoid  doing  violence  to  the  opinions  of  either  faction. 

The  home  of  the  tinamous  (Fig.  157,  A),  of  which  there  are  about 
forty  species,  is  the  New  World,  their  range  being  from  the  extreme 
lower  end  of  South  America  to  Mexico.  Though  they  bear  a  strong 
superficial  resemblance  to  gallinaceous  birds,  especially  the  partridges, 
it  is  believed  by  some  authorities  that  they  are  more  fundamentally 
related  to  the  Ratitae,  though  they  are  able  to  fly.  Their  wings  are 
short  and  rounded,  but  the  keel  of  the  sternum  is  well  developed  and 
the  pectoral  musculature  is  large  in  size.  The  tail  feathers  are  re- 
duced in  size,  even  rudimentary  in  some  species.  They  are  strong, 
swift  runners  and  are  reluctant  to  resort  to  real  flight;  nevertheless 
when  they  do  fly  they  make  a  fairly  good  job  of  it  for  short  distances. 
With  a  great  whirring  of  wings  and  extraordinary  effort  they  rise  to 
fifty  or  sixty  yards  above  the  ground  and  then  with  expanded  wings 
glide  slowly  down  to  the  ground,  covering  distances  of  about  a  thou- 
jand  yards,  which  may  be  repeated  several  times  if  necessary. 

It  seems  likely  that  the  tinamous  represent  a  condition  intermedi- 
ate between  the  flying  birds  proper  and  the  running  or  flightless  birds, 
for  they  possess  characters  that  relate  them  to  both  groups.  Some 
authorities  believe  that  the  ostriches  and  their  kin  have  probably 
been  derived  from  an  early  group  of  rather  weak  fliers  that  is  now 
represented  by  the  modern  tinamous. 

KEELED  OR  FLYING  BIRDS  (NEORNITHES  CARINAT^) 

Nearly  twelve  thousand  species  of  modern  birds  belong  to  this 
great  division,  as  compared  with  a  dozen  or  so  species  of  all  other 
living  birds.  The  study  of  birds  has  grown  into  the  highly  specialized 
science  of  Ornithology,  and  a  very  large  number  of  both  professional 
and  amateur  naturalists  and  bird  lovers  have  been  engaged  in  adding 
to  the  already  voluminous  annals  of  bird  lore  and  pseudo-lore.  A 
vast  literature  dealing  with  the  habits,  distribution,  migrations,  and 
adaptations  of  birds  has  accumulated,  much  of  which  is  worthless,  be- 
cause exaggerated,  inaccurate,  and  superficial.  But  the  authentic 
literature  on  all  phases  of  bird  life  is  so  voluminous  that  no  one  but  a 
specialist  can  hope  to  keep  abreast  of  it. 

The  classification  of  the  carinate  birds,  though  elaborate,  is  in  a 
fairly  satisfactory  condition.  Only  in  a  few  minor  points  is  there 
radical  disagreement  among  authorities.  Among  the  most  acceptable 


AVES  289 

classifications  of  the  Carinatse  is  that  of  Knowlton  in  his  "Birds  of 
the  World/'  and  that  of  Evans  in  the  volume  on  " Birds"  in  the 
Cambridge  Natural  History,  an  outline  of  which  is  as  follows: 

Brigade  I.  (Largely  archaic  types) 

LEGION  I.       COLYMBIMORPH^E     (Diver-like  Birds) 
Order       1.  Ichthyornithiformes 

2.  Colymbiformes 

3.  Sphenisciformes 

4.  Procellariiformes 

LEGION  II.    PELARGOMORPH.E     (Stork-like  Birds) 
Order      5.  Ciconiiformes 
6.  Anseriformes 
"  7.  Falconiformes 

Brigade  II.  (Largely  modern  types) 

LEGION  III.  ALECTOROMORPH^I     (Fowl-like  Birds) 
Order        8.  Tinamiformes 
"  9.  Galliformes 

"          10.  Gruiformes 

11.  Charadriiformes 

LEGION  IV.  CORACIOMORPH^    (Crow-like  Birds) 
Order       12.  Cuculiformes 

13.  Coraciiformes 

14.  Passeriformes 

Knowlton 's  classification  differs  from  that  of  Evans  in  only  a  few 
major  particulars: —  1,  the  tinamous  are  placed  among  the  Ratitae 
in  close  association  with  the  ostriches,  instead  of  next  to  the  Galli- 
formes; 2,  the  penguins  are  placed  among  the  flightless  birds,  im- 
mediately following  the  Ratitae;  the  genus  Ichthyornis  is  placed  in  the 
same  order  as  the  toothed  diving  birds  Hesperornis  and  Baptornis 
instead  of  among  the  flying  or  carinate  birds;  4,  there  is  no  brigading 
of  the  birds  into  archaic  and  modern  brigades,  and  there  is  no  group- 
ing of  orders  into  legions,  a  proceeding  that  is  less  likely  to  lead  into 
false  phylogenetic  implications  than  the  somewhat  artificial  grouping 
of  Evans. 

The  present  writer  prefers  to  follow  neither  classification  rigidly 
but  to  use  a  combination  of  the  two  methods.  The  limitations  of  the 


290 


VERTEBRATE  ZOOLOGY 


present  volume  forbid  any  but  the  most  cursory  treatment  of  this 
immense  and  extremely  attractive  assemblage  of  modern  verte- 
brates. Our  plan  will  be  to  give  a  brief  characterization  of  each  order, 
to  indicate  the  various  groups  that  comprize  it,  and  to  select  a  few 


FIG.  155.  —  Ichthyornis  victor.    Restored  skeleton. 
after  Marsh.) 


(From  Parker  and  Haswel 


of  the  most  interesting  or  significant  species  for  illustrative  types. 
In  many  cases  the  species  singled  out  for  description  are  of  no 
more  intrinsic  interest  or  importance  than  many  others  that  might 


AVES  291 

have  been  chosen.  As  a  rule  when  the  choice  has  lain  between  a 
native  and  a  foreign  type  the  native  type  has  been  given  the  prefer- 
ence. 

TOOTHED  FLYING  BIRDS  (ICHTHYORNITHIFORMES) 

This  order  is  represented  by  the  single  extinct  genus  Ichthyornis 
(Fig.  155),  several  species  of  which  have  been  found  in  Cretaceous 
strata.  These  birds  were  evidently  rather  gull-like  divers,  if  one  may 
judge  by  structure,  but  differed  from  all  modern  carinate  birds  in  that 
they  had  true  teeth  in  sockets.  Were  it  not  that  they  are  distinctly 
keeled  or  flying  birds  they  might  appropriately  have  been  placed,  as 
Knowlton  places  them,  in  the  order  with  the  toothed  diving  birds  of 
even  earlier  times. 

THE  PENGUINS  (SPHENISCIFORMES) 

These  curious,  highly  specialized,  marine  diving  birds  (Fig.  156,  B), 
have  a  wide  distribution  among  the  Antarctic  Seas.  They  are  really 
flightless  birds  and  might  on  that  account  be  excluded  from  the  Car- 
inatse,  but  they  have  well-developed  wings  and  a  fairly  good  keel  to 
the  sternum,  the  wings  being  used  for  "flying"  through  the  water 
instead  of  through  the  air;  for  the  wings  and  not  the  feet  are  the 
chief  organs  of  locomotion;  a  unique  character  among  diving  birds. 
The  legs  of  the  penguin  are  set  so  far  back  on  the  trunk  that  in  the 
water  they  are  used  primarily  as  a  rudder,  and  on  land  their  terminal 
position  makes  the  bird  practically  sit  upright  on  the  tail.  The  wings 
are  modified  into  flippers  not  unlike  those  of  the  whale;  they  are  quite 
devoid  of  flight  feathers  and  the  bony  framework  is  quite  stiff  and 
inflexible.  The  swimming  stroke,  when  under  the  water,  consists  of 
alternating  rotary  sweeps  of  the  two  flipper-like  wings,  which  drive 
the  pointed  body  through  the  water  at  a  fine  speed.  Penguins  live 
on  fish,  mollusks,  and  crustaceans.  They  are  markedly  gregarious, 
especially  during  the  breeding  season,  thousands  of  them  being  con- 
gregated upon  the  narrow  confines  of  rocky  islets  and  points  of  land 
along  the  sea  shores.  From  various  elevations  they  are  constantly 
diving  into  the  icy  water  after  their  food,  emerging  wet  and  glistening, 
but  capable  of  almost  instantly  drying  their  plumage  by  vigorous 
shaking  of  the  muscular  skin.  The  penguins  and  the  screamers  are 
the  only  birds  that  have  the  skin  completely  covered  with  feathers. 
In  the  penguins  the  feathers  are  lance-shaped  and  have  flattened 


292  VERTEBRATE  ZOOLOGY 

shafts;  they  overlap  one  another  in  the  most  perfect  fashion  so  as  to 
shed  effectively  all  water  from  the  skin.  Certain  burrowing  species  of 
the  Falkland  Islands  differ  rather  sharply  from  the  others,  in  that 
they  lay  their  eggs  in  rather  shallow  burrows.  The  penguins  are  con- 
sidered to  be  so  radically  different  in  structure  from  both  flying  birds 
and  ratite  birds  that  they  might  well  be  placed  in  a  separate  division 
coordinate  in  rank  with  the  Ratitse  and  the  Carinatse. 

THE  LOONS  AND  GREBES  (COLYMBIFORMES) 

This  archaic  and  quite  isolated  group  of  diving  birds  is  placed 
first  among  the  modern  flying  birds  because  they  possess  a  more 
generalized  structure  than  any  other.  The  loon  or  great  northern 
diver  (Fig.  156,  A),  is  the  example  of  the  order  most  familiar  to  dwell- 
ers in  the  Northern  States.  Its  weird,  laughing  cry  is  one  of  the  out- 
standing features  of  our  northern  woodland  life.  The  ability  of  the 
loon  to  dodge  a  bullet  by  diving  is  proverbial,  even  if  not  true.  The 
coloration  of  this  striking  bird  is  a  study  in  contrasting  blacks  and 
whites,  with  a  checkered  pattern  on  the  back,  white  breast,  black 
head,  and  white  and  black  bands  on  the  neck.  On  the  land  the  loon 
is  quite  clumsy  and  makes  poor  progress  in  walking.  It  really  never 
seems  to  come  ashore  except  for  nesting  purposes,  when  it  deposits  its 
two  large  eggs  in  some  slight  depression  not  far  from  the  water's  edge. 
Fortunately,  the  eggs  are  of  a  brownish  mottled  color  and  are  so 
nearly  in  harmony  with  the  background  that  they  are  very  difficult  to 
detect. 

Another  species  of  loon,  the  Pacific  Loon,  has  been  studied  by 
Coues,  who  gives  the  following  realistic  description  of  its  behavior 
in  the  water: 

"Now  two  or  three  would  ride  lightly  over  the  surface,  with 
neck  gracefully  curved,  propelled  with  idle  strokes  of  their  pad- 
dles to  this  side  and  that,  one  leg,  often  the  other,  stretched  at 
ease  almost  horizontally  backward,  while  their  flashing  eyes,  first 
directed  upward  with  sidelong  glance,  then  peering  into  the  depths 
below,  sought  for  some  attractive  morsel.  In  an  instant,  with  a 
peculiar  motion,  impossible  to  describe,  they  would  disappear 
beneath  the  surface,  leaving  a  little  foam  and  bubbles  to  mark 
where  they  went  down,  and  I  could  follow  their  course  under  the 
water;  see  them  shoot  with  marvelous  swiftness  through  the 
liquid  element,  as,  urged  by  the  powerful  strokes  of  the  webbed 


AVES 


293 


D  ^%E~r 


F 


FIG.  156. — Archaic  Carinate  Birds.  A,  Loon,  or  Great  Northern  Diver, 
Colymbus  gracialis;  B,  Rock-hopper  Penguin,  Eudyptes  chrysocoma;  C,  Albatross, 
Diomedea  exulans;  D,  White  Stork,  Ciconia  alba;  E,  Red-breast  Goose,  Bernicla 
ruficollis;  F,  Red  Kite,  Milvus  ictinius.  (D  and  D  after  Lydekker,  the  rest  after 

Evans.) 


294  VERTEBRATE  ZOOLOGY 

feet  and  beats  of  the  half -opened  wings,  they  flew  rather  than 
swam;  see  them  dart  out  the  arrow-like  bill,  transfix  an  unlucky 
fish,  and  lightly  rise  to  the  surface  again." 

Loons  are  almost  as  efficient  as  flyers  as  they  are  as  divers  and 
swimmers;  in  this  respect  they  are  much  more  generalized  than  are 
the  penguins. 

The  grebes  are  somewhat  more  like  penguins  than  are  the  loons, 
though  they  too  are  good  flyers.  They  are  much  smaller  than  loons 
and  have  a  much  wider  distribution,  being  practically  cosmopolitan 
in  their  range.  The  European  little  grebe,  or  "dabchick,"  is  an  in- 
teresting little  fellow  about  nine  inches  in  length.  It  has  attracted  a 
good  deal  of  attention  on  account  of  its  unique  habit  of  taking  its 
young  one  under  its  wing  when  diving  into  the  water  to  escape  from 
its  enemies  on  the  land  or  in  the  air.  The  American  Eared  Grebe 
is  characterized  by  the  presence  of  conspicuous  tufts  of  feathers  on 
the  sides  of  its  head  that  look  like  ears.  The  great  crested  grebe  and 
the  pied-billed  grebe,  which  is  an  American  dabchick,  are  two  other 
well-known  species. 

THE  PETRELS  AND  ALBATROSSES  (PROCELLARIIFORMES) 

These  sea  birds  are  characterized  by  powers  of  flight  more  marked 
than  any  other  group.  Because  of  their  ability  to  travel  great  dis- 
tances with  the  greatest  ease  they  have  attained  a  world-wide  dis- 
tribution. 

The  petrels  are  birds  of  moderate  size,  with  extremely  long,  narrow 
wings  and  hooked  bill.  They  soar  about  over  the  waves  and  dive 
into  the  sea  after  fish,  their  main  food.  The  stormy  petrel  is  consid- 
ered by  mariners  as  a  prophet  of  rough  weather  when  it  hovers 
about  ships  at  sea. 

The  albatross  (Fig.  156,  C),  is  one  of  the  largest  of  flying  birds, 
considerably  larger  than  a  goose.  The  following  vivid  word  picture 
of  Professor  Hutton  will  serve  to  acquaint  the  reader  with  one  of  our 
noblest  birds: 

"With  outstretched,  motionless  wings  he  sails  over  the  surface 
of  the  sea,  now  rising  high  in  the  air,  now  with  a  bold  sweep,  and 
wings  inclined  at  an  angle  with  the  horizon,  descending  until  the 
tip  of  the  lower  one  all  but  touches  the  crests  of  the  waves  as  he 
skims  over  them.  Suddenly  he  sees  something  floating  on  the 
water  and  prepares  to  alight;  but  how  changed  he  now  is  from  the 


AVES  295 

noble  bird  but  a  moment  before  all  grace  and  symmetry.  He 
raises  his  wings,  his  head  goes  back,  and  his  back  goes  in;  down 
drop  two  enormous  webbed  feet  straddled  out  to  their  full  extent, 
and  with  a  hoarse  croak,  between  the  cry  of  a  Raven  and  that  of 
a  sheep,  he  falls  '  souse '  into  the  water.  Here  he  is  at  home  again, 
breasting  the  waves  like  a  cork.  Presently  he  stretches  out  his 
neck,  and  with  great  exertion  of  his  wings  runs  along  the  top 
of  the  water  for  seventy  or  eighty  yards,  until,  at  last,  hav- 
ing got  sufficient  impetus,  he  tucks  up  his  legs,  and  is  once  more 
fairly  launched  into  the  air." 

Several  less  well-known  types  of  bird  are  also  classed  in  this  order: 
fulmars,  shearwaters,  and  diving  petrels. 

THE  STORK-LIKE  BIRDS  (CICONIIFORMES) 

This  rather  mixed  assemblage  of  water  birds  includes:  tropic  birds, 
gannets,  cormorants,  darters,  frigate  birds,  pelicans,  bitterns,  herons, 
ibises,  storks,  spoonbills,  flamingoes,  etc.  So  extensive  and  varied 
a  group  is  it  that  it  is  difficult  to  characterize  it  as  a  whole.  It  is 
subdivided  into  eleven  families,  most  of  which  are  birds  capable  of 
sustained  flight;  many  of  them  are  wading  rather  than  swimming 
birds. 

The  tropic  birds  are  true  denizens  of  the  tropic  oceans,  flying 
hundred  of  miles  from  land,  and  taking  shelter  and  rest  amid  the 
rigging  of  ships  when  opportunity  affords.  Gannets  are  also  sea  birds, 
but  frequent  the  colder  regions,  coming  ashore  during  stormy  weather. 
Cormorants  are  rather  large  sea-coast  birds  with  pronounced  fish- 
eating  proclivities;  they  are  not  exclusively  marine,  but  frequent 
inland  lakes  during  the  breeding  season.  Darters  or  snake  birds  are 
not  marine,  but  frequent  inlets  of  the  sea  and  fresh-water  lakes;  they 
are  not  very  strong  flyers,  but  excel  as  divers,  leaving  scarcely  a  ripple 
upon  the  surface  when  they  go  down  after  fish. 

The  frigate  bird  or  man-of-war  bird  is  a  true  sea-bird  seldom 
coming  ashore  except  to  nest.  The  long  wings  and  extremely  long 
tail  are  distinctive  features.  Pelicans  are  familiar  large  birds  of  the 
tropics.  The  bill  is  very  large  and  the  lower  jaw  is  provided  with  a 
capacious  pouch  in  which  a  large  supply  of  captured  fish  can  be  stored. 
The  stubby  tail  and  short  legs  are  familiar  attributes  of  this  interest- 
ing bird.  Herons,  ibises  and  storks  (Fig.  156,  D)  have  a  strong  gen- 
eral resemblance;  their  long  legs,  collapsible  necks,  and  slow  flapping 


296  VERTEBRATE  ZOOLOGY 

wing-stroke  are  known  to  every  child.  The  spoonbill  is  a  stork-like 
bird  with  a  spoon-shaped  bill  with  which  it  captures  insect  prey, 
larvae,  fish,  frogs,  etc. ;  they  are  largely  tropical  in  distribution. 
Flamingoes  are  large,  extremely  long-legged,  long-necked  birds,  with 
wonderful  pink  plumage.  They  are  good  flyers,  but  are  more  char- 
acteristically waders.  In  the  breeding  season  they  are  decidedly 
gregarious,  building  extensive  colonies  of  tall,  chimney-like  nests  of 
mud,  that  are  hollowed  out  at  the  top  to  receive  the  eggs.  These 
nests,  which  look  like  a  lot  of  tree-stumps,  are  made  high  partly  to 
keep  the  eggs  out  of  reach  of  the  water,  for  they  are  built  on  low 
ground,  and  partly  because  they  are  of  a  convenient  height  for  these 
long-legged  creatures  to  sit  down  upon.  One  might  imagine  the 
rather  precarious  situation  involved  in  an  attempt  of  these  stilted 
birds  to  sit  down  on  a  nest  built  at  the  level  of  the  ground.  In  certain 
respects  the  flamingoes  are  transitional  between  the  storks  and  the 
geese. 

THE  GOOSE-LIKE  BIRDS  (ANSERIFORMES) 

This  order  is  divided  into  two  quite  well-defined  sub-orders  con- 
sisting of  the  screamers  and  the  Anseres  proper. 

The  screamers  are  quite  unlike  the  goose  tribe  in  general  appear- 
ance and  in  habits,  and  it  is  only  on  the  basis  of  skull  and  skeletal 
characters  that  they  are  classed  as  Anseriformes.  They  are  about  the 
size  of  turkeys  and  have  a  fowl-like  head  and  bill.  They  are  highly 
unique  in  two  respects;  the  ri&s  are  entirely  devoid  of  uncinate  proc- 
esses, which  are  possessed  by  all  other  living  birds;  and  they  share 
with  the  penguins  and  ratites  the  distinction  of  being  the  only  birds 
having  the  entire  skin  covered  with  feathers,  no  apteria  or  naked  areas 
being  present.  Some  writers  consider  these  two  characters  so  dis- 
tinctive that  they  would  assign  to  the  screamers  rank  as  a  separate 
order. 

The  horned  screamer  is  the  best  known  species,  characterized  by 
the  possession  of  a  forward  curving  brow-horn  about  five  inches  in 
length.  It  also  has  on  the  anterior  margin  of  each  wing  two  sharp 
claw-like  spikes,  that  could  readily  do  considerable  damage  to  an 
antagonist.  The  disproportionately  loud  screaming  note  of  these 
strange  birds  has  given  them  their  name. 

The  remaining  members  of  the  order  are  Anseres,  familiar  types  to 
everyone.  The  swans  are  large  birds  emblematic  of  grace  of  form 


AVES  297 

and  movement.  The  geese  proper  are  the  most  generalized  members 
of  the  order,  and  are  intermediate  between  the  swans  and  the  ducks 
in  their  characters,  especially  in  the  length  of  the  neck.  Some  of  the 
ducks  are  among  the  most  brilliant  in  plumage  among  birds.  Few 
handsomer  vertebrates  exist  than  the  male  mandarin  duck.  The 
eider  ducks  are  natives  of  the  north  and  are  the  most  widely  known 
and  highly  prized  members  of  the  duck  family.  The  mergansers  or 
fish-ducks  differ  from  the  true  ducks  in  having  more  slender  bodies, 
long  compressed  bills,  and  grebe-like  necks,  and  in  having  the  edges  of 
the  bill  serrated  so  as  to  give  the  impression  that  they  have  teeth. 
On  account  of  their  fish-eating  habits  they  are  not  nearly  so  desirable 
for  food  as  are  most  of  the  ducks  and  geese,  which  are  largely  gram- 
eniferous. 

FALCON-LIKE  BIRDS  OR  BIRDS  OF  PREY  (FALCONIFORMES) 

Just  as  the  great  carnivores  among  mammals  are  designated  as  the 
" kings  of  beasts,"  so  the  great  birds  of  prey  (eagles,  hawks,  falcons, 
etc.)  are  " kings  among  birds."  The  members  of  this  order  are  char- 
acterized by  hooked,  raptorial  beak,  strong  talons,  large  crop  and 
predaceous  habits.  So  much  are  the  eagles  objects  of  human  admira- 
tion that  they  have  been  chosen  as  emblems  of  empire;  even  our 
own  naturally  peaceful  commonwealth  is  proud  to  be  represented  by 
the  king  of  American  eagles.  The  order  Falconiformes  falls  into 
three  subdivisions,  represented  respectively  by:  the  American  vul- 
tures; the  secretary  bird  of  Africa;  and  the  falcons,  eagles,  hawks, 
buzzards,  Old  World  vultures,  etc . 

The  American  vultures  are  large  birds  with  wonderful  powers  of 
flight,  though  somewhat  sluggish  in  habit.  The  common  turkey 
buzzard  is  the  most  conspicuous  example  of  this  division  and  is 
a  familiar  part  of  the  scenery  in  most  of  our  Southern  States.  They 
are  economically  of  considerable  importance  on  account  of  their 
effective  work  as  scavengers,  and  on  this  account  there  are  laws  pro- 
tecting them  from  marksmen.  In  spite  of  their  value  as  sanitary 
agents  they  are  generally  looked  down  upon  because  of  their  disgust- 
ing feeding  habits  and  because  they  have  a  trick  of  vomiting  upon 
their  adversaries.  If  the  truth  were  known  it  would  probably  be 
found  that  the  buzzard  is  a  victim  of  chronic  dyspepsia  due  to  the 
unwholesome  character  of  its  food,  and  that  it  accepts  with  gratitude 
any  offerings  of  fresh  meat  that  may  come  its  way.  It  is  said  to  eat 


298  VERTEBRATE  ZOOLOGY 

carrion  because  its  beak  is  not  strong  enough  to  enable  it  to  kill  living 
prey.  Perhaps  the  poor  buzzard  is  more  to  be  pitied  than  censured. 
The  Andean  condor,  and  the  California  condor,  and  the  king  vulture 
are  other  familiar  members  of  the  present  group. 

The  secretary  bird  (Gypogeranus)  is  perhaps  the  strangest  of 
all  birds  of  prey.  It  is  a  long-legged  bird,  rather  more  like  a  crane  in 
proportions  than  like  the  other  members  of  its  order;  it  stands  about 
four  feet  in  height  on  long  slender  legs,  upon  which  it  places  more 
reliance  for  speeding  than  upon  its  wings.  It  is  especially  fond  of 
snakes,  though  it  accepts  lizards,  frogs  and  insects.  Its  method  of 
attacking  a  snake  is  unique.  The  snake  is  incited  to  strike,  and  when 
it  does  the  bird  side-steps  and  receives  the  blow  on  the  edge  of  its 
stiffly  extended  wing.  The  force  of  the  blow  seems  to  stun  the  snake 
momentarily,  and  the  bird  pounces  on  it  and  grasps  it  by  the  neck 
with  its  powerful  talons. 

The  remaining  subdivision  includes  the  following  types:  falcons, 
gyrfalcons,  duck-hawks,  kestrels,  falconets,  carrion-buzzards,  nu- 
merous types  of  hawks,  caracaras,  true  eagles,  hawk-eagles,  harpies, 
harriers,  and  Old  World  vultures.  Several  other  less  known  types 
might  be  mentioned,  but  these  will  suffice. 

The  golden  eagle  may  well  be  allowed  to  represent  the  entire 
collection.  This  characteristic  American  bird  is  nearly  a  yard  long 
and  has  a  wing-spread  of  nearly  seven  feet.  It  is  proverbial  for  its 
courage,  but  one  is  somewhat  taken  aback  by  what  Major  Bendire 
says  about  it: — "Notwithstanding  the  many  sensational  stories  of 
the  fierceness  and  prowess  of  the  golden  eagle,  especially  in  defense 
of  its  eyrie,  from  my  own  observations  I  must  confess,  if  not  an  ar- 
rant coward,  it  certainly  is  the  most  indifferent  bird,  in  respect  to  the 
care  of  its  eggs  and  young,  I  have  ever  seen."  This  disclosure  might 
possibly  make  us  doubt  the  wisdom  of  our  selection  of  a  national  em- 
blem, but,  as  though  to  compensate  its  faults  in  some  respects,  it  is 
given  credit  for  being  "a  clean,  trim-looking,  handsome  bird,  keen- 
sighted,  rather  shy  and  wary  at  times,  even  in  thinly  settled  parts  of 
the  country,  swift  of  flight,  strong  and  powerful  of  body,  and  more 
than  a  match  for  any  animal  of  similar  size."  To  say  the  very  least 
the  bird  is  efficient,  and  in  this  respect  not  so  bad  an  emblem.  Let 
him  who  will  f*»>d  a  better  bird ! 


AVES  299 

THE  FOWL-LIKE  BIRDS  (GALLIFORMES) 

This  order  is  a  large  and  cosmopolitan  one  and  is  divided  into  four 
sub-orders,  three  of  which  are  small  and  the  other  contains  all  of  the 
numerous  types  of  game  birds. 

The  Magagascar  mesite  is  the  sole  representative  of  the  first 
sub-order  (Mescenatides).  It  is  decidedly  aberrant,  having  a  head  and 
bill  more  like  that  of  a  rail  than  like  that  of  a  game  bird.  So  anoma- 
lous is  this  bird  that  various  authorities  have  classed  it  respectively 
with  the  rails,  with  the  cranes,  and  even  with  the  song-birds. 

The  Hemipodes  or  bustard  quails  (Turnices),  representing  the 
second  sub-order,  are  in  outward  appearance  not  unlike  small  quails  or 
partridges,  but  differ  so  fundamentally  from  the  latter  in  skeletal 
structure  that  they  are  placed  in  a  separate  division. 

The  Gallinaceous  game  birds  (Galli)  comprise  a  large  assem- 
blage of  more  or  less  familiar  types,  most  of  which  need  no  descrip- 
tion. Apart  from  the  game  birds  proper  the  Galli  include  two  families 
of  unfamiliar  birds,  represented  by  the  brush  turkeys  (Megapodes), 
of  Australia  and  New  Guinea,  and  the  curassows  and  guans  (Cracidce) 
of  tropical  America. 

The  true  Gallinaceous  game  birds  consist  of  wild  turkeys,  guinea- 
fowls,  grouse,  partridges,  quails,  ptarmigans,  prairie-hens,  bob- 
whites,  pheasants,  jungle-fowls  and  pea-fowls.  The  most  highly 
specialized  types  of  Galli  are  characterized  by  most  gorgeous  plumage, 
notable  example  being  the  males  of  the  golden  and  Lady  Amherst 
pheasants,  which  are  native  to  South  China  and  Eastern  Thibet. 
As  described  by  Mr.  Ogilvie-Grant,  the  male  of  the  golden  pheasant 
has  the  top  of  the  head,  crest  and  rump  brilliant  golden  yellow,  the 
square-tipped  feathers  of  the  back  and  neck  brilliant  orange,  tipped 
and  banded  with  steel  blue,  while  the  throat  and  sides  of  the  head  are 
pale  rust  color,  the  shoulders  and  remainder  of  the  under  parts  crim- 
son-scarlet, and  the  middle  tail  feathers  black  with  rounded  spots  of 
pale  brown;  the  tail  is  twenty-seven  out  of  a  total  length  of  forty 
inches.  If,  as  we  have  maintained,  extravagance  of  coloration  is  one 
of  the  criteria  of  racial  senescence,  this  is  one  of  the  most  senescent  of 
the  birds.  The  pea-fowls  are  almost  as  wonderfully  colored  as  the 
finest  of  the  pheasants,  but  they  are  too  familiar  to  require  de- 
scription. Their  native  home  is  in  oriental  countries,  but  they  have 
been  domesticated  and  widely  distributed  by  man. 


'300  VERTEBRATE  ZOOLOGY 

The  jungle-fowls  deserve  special  mention  because  it  is  from 
them  that  our  domestic  poultry  have  been  derived.  Four  distinct 
species  of  jungle-fowl  are  known,  all  of  them  native  to  the  dense 
jungles  of  the  Indo-Malayan  region.  Of  these  it  is  believed  that  the 
red  jungle-fowl  (Gallus  gallus)  has  given  rise  to  all  of  the  domestic 
breeds  of  poultry.  The  breed  known  as  the  black-breasted  game  has 
retained  more  completely  than  any  of  the  others  the  characters  of  the 
wild  ancestor.  The  most  extreme  deviations  from  the  primitive 
characters  of  the  species  are  seen  in  the  Japanese  tosa  fowl,  in  which 
the  tail  feathers  have  been  known  to  reach  a  length  of  fifteen  feet, 
and  the  cochins,  with  their  short,  plump  appearance  and  feathered 
shanks. 

The  hoactzin,  representing  the  fourth  sub-order  (Opisthocomi) , 
is  one  of  the  most  curious  of  birds.  In  the  adult  condition  it  is  not 
unlike  a  small  type  of  pheasant,  but  it  has  certain  anatomical  char- 
acters that  set  it  apart  from  all  other  birds: — the  breast  bone  is  wider 
behind  than  in  front;  the  keel  of  the  sternum  is  confined  to  the  poste- 
rior part;  the  crop  is  extremely  large  and  muscular,  invading  the  space 
usually  taken  up  with  pectoral  muscles  and  the  anterior  part  of  the 
sternum;  and  the  bones  of  the  shoulder  girdle  are  fused  completely 
to  one  another  and  to  the  sternum.  The  most  remarkable  features  of 
the  hoactzin,  however,  concern  the  young  bird,  which,  when  newly 
hatched,  has  a  well-developed  clawed  thumb  and  index  finger  on  the 
wing,  reminding  one  of  the  condition  in  Archceopteryx.  By  means  of 
these  wing-digits  and  the  feet  which  are  extraordinarily  large  and 
strong  for  a  young  bird,  these  youngsters  are  able  to  clamber  about 
among  the  branches  and  hunt  for  their  own  food.  They  are  really 
practically  quadrupedal  in  the  use  of  both  pairs  of  limbs  in  climbing. 
It  is  believed  by  some  writers  that  the  juvenile  characters  of  the  hoact- 
zin are  reminiscences  of  an  Archseopteryx-like  ancestry.  Inasmuch, 
however,  as  they  belong  to  one  of  the  more  highly  specialized  groups 
of  birds,  and  inasmuch  as  no  other  known  type  of  bird  exhibits 
similar  juvenile  characters,  it  seems  more  likely  that  these  characters 
are  adaptive,  juvenile  specializations.  In  view  of  all  of  the  peculiari- 
ties of  the  hoactzins  it  is  difficult  satisfactorily  to  classify  them  in 
any  order;  but  the  rather  strong  resemblance  of  the  adult  to  the 
pheasants  seems  to  place  them  among  the  Galliformes. 


AVES  301 

THE  CRANE-LIKE  BIRDS  (GRUIFORMES) 

The  majority  of  the  members  of  this  order  are  waders,  but  some, 
such  as  the  bustards  and  the  wekas,  are  decidedly  terrestrial.  The 
group  does  not  hold  together  as  well  as  some  of  the  others,  and  prob- 
ably should  be  divided  into  two  orders.  Seven  families  are  distin- 
guished, represented  by  the  following  types: — rails,  gallinules,  and 
coots;  bustards;  the  kagu;  sun-bitterns;  and  finfoots. 

The  common  sandhill  crane  is  probably  the  most  abundant  and 
conspicuous  example  of  the  larger  Gruiformes  in  America.  Coues, 
much  impressed  by  their  appearance  in  migration  flight,  writes  of 
them  as  follows: — 

"Such  ponderous  bodies,  moving  with  slow-beating  wings, 
give  a  great  idea  of  momentum  from  mere  weight — of  force  of 
motion  without  swiftness;  for  they  plod  along  heavily,  seeming 
to  need  every  inch  of  their  ample  wings  to  sustain  themselves. 
One  would  think  they  must  soon  alight  fatigued  with  such  exer- 
tion, but  the  raucous  cries  continue,  and  the  birds  fly  on  for 
miles  along  the  tortuous  stream,  in  Indian  file,  under  some  trusty 
leader,  who  croaks  his  hoarse  orders,  implicitly  obeyed." 
The  great  bustard  is  the  largest  European  bird,  being  about 
forty-five  inches  long  and  weighing  nearly  thirty  pounds.    In  general 
appearance  it  is  not  unlike  a  goose,  but  has  a  head  and  bill  more  like 
that  of  a  crane.    Sun-bitterns  are  rather  small  birds  something  like  a 
rail  and  a  heron,  but  with  rather  short  legs,  a  very  thin  neck  and  large 
head  with  long  pointed  bill .    When  at  rest  the  head  is  sunk  down  on 
the  body  so  as  to  give  it  the  appearance  of  being  practically  neckless. 
The  finfoot  tribe  consists  of  birds  about  whose  relationship  there  is  a 
good  deal  of  controversy;  some  authorities  placing  them  among  the 
grebes,  on  account  of  the  grebe-like  head  and  bill.     The  rails  are 
rather  ordinary  birds,  so  far  as  appearance  goes;  but  they  are  of  in- 
terest because  they  are  believed  to  be  intermediate  between  the  two 
orders  Galliformes  and  Charadiiformes.     In  general  appearance  they 
remind  one  of  both  the  quail  and  the  plover. 

THE  PLOVER-LIKE  BIRDS  (CHARADRIIFORMES) 

This  order  is  considerably  more  homogeneous  than  the  last,  but 
it  is  difficult  to  select  a  good  popular  name  for  the  group;  for  the 
gulls  and  pigeons  are  in  truth  not  very  "  plover-like."  The  plovers 


302  VERTEBRATE  ZOOLOGY 

are  the  most  generalized  members  of  the  order  and  may  well  give  to  it 
their  name.  There  are  four  sub-orders :  Limicolce  (typical  plover-like, 
shore-feeding  birds),  Lari  (the  gulls  and  terns),  Pterodes  (the  sand 
grouse),  and  Columbce  (the  pigeons). 

The  Limicolae  are  marsh  and  shore  birds,  with  fairly  long  neck, 
long  slender  bill,  legs  moderately  long  and  slender,  short  tail  and 
wings,  and  plumage  streaked  and  of  inconspicuous  patterns.  They 
usually  nest  on  the  ground  and  the  young  are  capable  of  running  very 
soon  after  hatching.  To  this  sub-order  belong:  the  plovers,  snipes, 
and  curlews;  the  sheath-bills;  the  crab-plovers;  the  pratinicoles  and 
coursers;  the  sand  snipes;  the  thick-knees;  and  the  jacanas.  Most  of 
these  are  birds  without  any  outstanding  characteristics  that  might 
capture  the  attention.  As  an  example  we  may  well  select  the  American 
woodcock  (Fig.  157,  D),  a  species  native  to  the  Mississippi  valley.  This 
bird  has  an  unusually  long  bill,  which  it  uses  largely  for  unearthing 
earthworms  from  their  burrows.  It  is  said  that  a  woodcock  will  eat 
half  a  pound  of  worms  in  a  day.  It  is  mainly  nocturnal  and  when 
flushed  in  the  daytime  appears  to  be  dazzled  by  the  light.  The 
jacanas  are  strange-looking  tropical  birds,  characterized  by  enormously 
long  toes  and  claws,  by  means  of  which  they  are  able  to  walk  about 
with  ease  over  the  lily-pads,  after  the  fashion  of  a  man  on  snow-shoes. 

The  Lari  are  the  gulls  and  their  allies,  a  group  almost  too  fa- 
miliar to  require  description.  They  are  aquatic,  mainly  oceanic,  in 
habitat,  are  of  medium  size  and  have  unusually  long,  pointed  wings. 
Besides  the  gulls,  terns,  noddies  and  such  typically  gull-like  birds  the 
sub-order  includes  the  auks,  the  puffins  and  the  murres.  The  puffins 
or  sea  parrots  are  the  most  grotesque  members  of  the  entire  order. 
They  have  a  brilliantly  colored,  laterally  compressed  bill;  and  their 
body  form  and  attitude  remind  one  of  that  of  the  penguins.  The 
great  auk,  a  recently  extinct  species,  is  of  considerable  interest.  Of 
it  Knowlton  says  that  "its  sad  and  untimely  fate  has  invested  it  with 
a  pathetic,  not  to  say  melancholy  history."  It  used  to  be  extremely 
abundant  on  the  islands  north  of  Scotland  and  near  Newfoundland, 
but  it  was  slaughtered  by  the  millions,  largely  for  its  feathers.  The 
eggs  were  also  collected  so  that  nothing  is  now  left  of  that  fine  species 
but  heaps  of  bones  scattered  about  the  lonely  islands.  The  last 
living  specimen  was  seen  in  1844. 

The  Pterocles  are  the  pigeon  grouse  or  sand  grouse,  a  small  group 
that  appears  to  combine  the  characters  of  several  orders  and  whose 


AVES 


303 


FIG.  157. — Representative  Carinate  Birds.  A,  Great  Tinamou,  Rhynchotus 
rufescens;  B,  Pheasant,  Phasianus  colchicus;  C,  Land-rail,  Crex  pratensis;  D, 
Woodcock,  Scolopax  rusticula;  E,  Hornbill,  Rhytidoceros  undulatus;  F,  Parrot, 
Psittacus  eriihacus.  (Redrawn  after  Evans.) 


304  VERTEBRATE  ZOOLOGY 

systematic  relations  are  not  at  all  certain.  Outwardly  they  appear 
to  be  intermediate  between  the  grouse  and  the  pigeon,  but  structur- 
ally they  more  nearly  approach  the  conditions  seen  in  the  pigeons. 

The  Columbae  include  the  pigeons  and  doves,  and  the  dodo  and 
solitaire.  The  dodo  is  a  recently  extinct,  aberrant,  not  to  say 
grotesque  and  gigantic  pigeon.  A  funnier  looking  bird  could  not 
readily  be  imagined,  if  we  may  credit  the  pictorial  records  of  it  made 
by  travelers  of  the  seventeenth  century.  That  these  apparent  cari- 
catures were  founded  on  fact  is  evidenced  by  the  bones  of  the  bird 
found  in  pools.  It  was  a  short,  plump  bird,  with  an  eagle-like  beak 
and  ridiculously  inadequate  plumage,  wings,  and  tail. 

The  true  pigeons  constitute  a  very  large  and  widely  distributed 
group.  Perhaps  the  most  interesting  and  significant  of  the  species 
are  the  rock  pigeon,  the  passenger  pigeon,  and  the  great  crowned 
pigeon. 

The  rock  pigeon  or  rock  dove  (Columbia  livid)  is  the  species  from 
which  nearly  all  of  the  fancy  breeds  of  domestic  pigeons  have  been 
derived;  and  when  fancy  breeds  are  allowed  to  interbreed  freely,  the 
offspring  tend  to  revert  to  the  characters  of  the  wild  ancestor.  The 
common  mongrel  pigeon  of  the  city  streets  represents  fairly  closely 
the  characteristics  of  the  wild  rock  pigeon.  The  passenger  pigeon  a 
century  ago  existed  in  numbers  almost  incredibly  large.  Wilson,  a 
pioneer  American  ornithologist,  estimates  that  in  a  single  flock  seen 
by  him  near  Frankfort,  Kentucky,  there  were  over  two  billion  indi- 
viduals. In  describing  similar  conditions,  Henderson  says  that  "the 
air  was  literally  filled  with  pigeons,  the  light  of  noonday  was  obscured 
as  by  an  eclipse,"  and  that  their  wings  made  "a  noise  like  thunder." 
"Nothing,"  says  Nuttall,  "can  exceed  the  waste  and  desolation  of  the 
nocturnal  resorts  (of  these  pigeons);  the  vegetation  becomes  buried 
by  their  excrement  to  the  depth  of  several  inches.  The  tall  trees  for 
thousands  of  acres  are  completely  killed,  and  the  ground  completely 
strewed  with  massive  branches  torn  down  by  the  clustering  weight  of 
the  birds  which  have  rested  upon  them.  The  whole  region  for  several 
years  presents  a  continued  scene  of  desolation,  as  if  swept  by  the  re- 
sistless blast  of  a  whirlwind."  At  the  present  time  it  is  a  question 
whether  there  are  any  passenger  pigeons  still  living.  An  isolated 
report  comes  in  now  and  then  that  someone  has  seen  a  specimen,  but 
there  is  usually  some  uncertainty  about  the  identification.  The  fate 
of  this  fine  species  of  bird  well  illustrates  the  ruthlessness  of  man  when 


AVES  305 

he  begins  the  process  of  extermination.  The  great  crowned  pigeon,  a 
native  of  the  Solomon  Islands,  represents  the  climax  of  the  evolution 
of  the  pigeon  family.  It  is  a  noble-looking  bird,  as  much  as  thirty- 
four  inches  in  length;  with  a  great,  erect,  fan-shaped  crest  of  feathers 
on  top  of  the  head  which  gives  it  a  regal  appearance. 

THE  CUCKOO-LIKE  BIRDS  (CUCULIFORMES) 

This  order  is  sharply  subdivided  into  two  sub-orders:  the  Cuculi 
(cuckoos  and  plantain-eaters),  and  the  Psittati  (parrots,  parrakeets, 
etc.). 

Of  the  Cuculi  Knowlton  says: — 

"Taking  everything  into  account,  the  Cuckoos  comprise  a 
very  remarkable  and  interesting  group  of  birds,  being  for  the 
most  part  birds  of  shams  and  pretenses,  and  ever  seeking  to  con- 
vey the  impression  that  they  are  other  than  they  really  are." 

We  might  well  call  them  "  camouflage  birds,"  a  term  that  would  well 
characterize  these  interesting  traits.  They  are  certainly  great  mimics 
both  of  the  appearance  and  of  the  voices  of  other  birds.  Some 
Cuckoos  place  their  eggs  in  the  nests  of  other  birds.  It  is  perhaps 
on  account  of  this  peculiar  parasitic  nesting  habit  that  they  are  best 
known.  Instead  of  building  a  nest  of  her  own,  the  female  lays  her  eggs 
on  the  ground  and  then  carries  them  in  her  bill  to  the  nest  of  other 
birds.  The  bird  thus  imposed  upon  is  likely  to  react  against  this 
intrusion  by  dumping  out  the  foreign  egg,  or  by  building  a  second 
story  to  the  nest,  thus  leaving  the  cuckoo  egg  walled  up  in  the  base- 
ment. Doubtless,  however,  a  sufficiently  large  number  of  cuckoo 
eggs  are  tolerated  by  other  birds  to  keep  up  the  normal  supply  of  the 
various  species.  This  parasitic  habit  belongs  to  the  Old  World 
cuckoos,  for  the  American  cuckoos  build  their  own  nests.  The  road- 
runner  is  an  interesting  terrestrial  Cuckoo  familiar  to  the  inhabitants 
of  the  Southwestern  United  States  and  Mexico.  One  sees  this  long- 
legged  bird  pacing  along  ahead  of  him  on  lonely  country  roads,  always 
keeping  a  respectful  distance  ahead,  but  not  offering  to  leave  the 
road  or  to  fly.  The  plantain-eaters  seem  to  be  in  some  ways  inter- 
mediate between  the  cuckoos  and  the  parrots. 

The  Psittaci,  parrots,  (Fig.  157,  F),  are  a  very  sharply  circum- 
scribed group  of  brilliant  and  interesting  birds.  There  are  over  eighty 
known  genera  but  they  are  all  unmistakably  related.  They  are 
usually  brilliant  in  plumage,  favoring  green,  yellow  and  brilliant  red 


306  VERTEBRATE  ZOOLOGY 

tints,  but  are  occasionally  brown  or  black.  They  are  climbing  arbo- 
real birds  that  use  the  bill  as  an  aid  to  climbing,  which  is  a  unique  use 
for  this  organ.  Perhaps  the  most  striking  characteristic  of  the  parrots 
is  their  ability  to  articulate.  Though  their  native  language  is  one  of 
discordant  screams,  they  can  be  taught  to  mimic  human  language 
with  moderate  success,  thus  showing  their  cuckoo-like  propensities  of 
pretending  to  be  what  they  are  not.  Among  the  parrots  are  included 
cockatoos,  parrakeets  and  macaws. 

ROLLER-LIKE  BIRDS  (CORACIIFORMES) 

This  is  one  of  the  largest  and  most  heterogeneous  of  the  avian 
orders;  having  affinities  with  the  cuckoo-like  birds,  on  the  one  hand, 
and  with  the  sparrow-like  birds,  on  the  other.  There  are  seven  sub- 
orders, most  of  which  are  not  literally  roller-like  in  appearance. 

The  Coraciae  (true  rollers  and  their  allies)  include:  rollers,  mot- 
mots  and  todies,  kingfishers,  bee-eaters,  horn-bills,  and  hoopoes; 
a  rather  motley  collection  of  types  in  itself.  The  common  roller  (Fig. 
158,  D)  is  a  native  of  Southern  Europe,  outwardly  resembling  many  of 
the  typical  passerine  birds.  The  horn-bills  (Fig.  157,  E)  are  the  most 
remarkable  of  the  Coracise;  they  are  large  birds  with  enormous  bill, 
used  by  the  male  as  a  trowel  in  the  operation  of  walling  up  the  female 
in  a  hollow  tree.  Whether  the  female  is  a  restless  sitter  and  needs 
thus  to  be  kept  on  the  job,  or  whether  the  wall  is  for  her  protection 
while  she  is  confined  at  her  intimate  task,  it  is  difficult  to  say.  She 
is  fed,  however,  by  the  male,  through  a  small  window  just  large  enough 
for  her  bill  to  be  thrust  out. 

The  Striges  (owls),  are  a  well-defined  group,  formerly  classed 
with  the  Falconiformes  on  account  of  their  predaceous  habits,  but 
now  known  to  have  closer  affinities  with  the  goat-suckers.  The  great 
horned  owl  (Fig.  158,  E)  is  the  finest  of  its  kind,  a  wise-looking, 
powerful  bird  of  great  size,  described  as  a  "  veritable  tiger  among 
birds."  It  kills  quails,  grouse,  doves,  wild  ducks,  as  well  as  all  sorts  of 
smaller  and  medium-sized  mammals.  It  hunts  at  night  and  hides  in 
hollow  trees  during  the  day.  The  little  American  screech  owl  is  the 
commonest  and  most  widely  distributed  of  our  owls. 

The  Caprimulgi  (goat-suckers  and  their  allies)  include  the  oil- 
bird,  the  frog-mouths,  the  goat-suckers  or  night  jars,  and  the  whip- 
poorwills.  They  are  all  much  alike,  being  characterized  by  rather 
compact  bodies,  very  short,  but  extremely  wide,  bill,  and  deeply 


AVES 


307 


FIG.  158. — Types  of  Coraciiformes,  showing  generalized  and  specialized  species. 
A,  Trogon  or  Quezal,  Pharomacrus  modnno;  B,  Toucan,  Rhamphastus  arid; 
C,  Hummingbird,  Eulampis  jugularis;  D,  Common  Roller,  Caracias  garrulus; 
E,  Great  Horned  Owl,  Bubo  virginianus.  (All  redrawn;  A,  B,  C,  after  Evans; 
D  and  E,  after  Knowlton.) 


308  VERTEBRATE  ZOOLOGY 

cleft  mouth  fringed  with  stiff  hairs,  used  to  trap  insects  as  they  fly 
through  the  air.  .  Their  flight  is  swift  and  practically  noiseless.  Their 
mournful  nocturnal  cries  sound  like  "  whip-poor-will,"  "  poor-will" 
"who-are-you,"  etc. 

Micropodii  (hummingbirds  and  swifts)  are  the  smallest  of  birds. 
Of  the  hummingbirds  (Fig.  158,  C)  much  has  been  written  in  praise 
of  their  beauty.  "Glittering  fragments  of  the  rainbow,"  Audobon 
calls  them;  while  Knowlton  characterizes  them  as  "gems  of  the  feath- 
ered race."  Small  though  they  be,  they  are  among  the  most  highly 
specialized  of  all  birds;  and  therefore,  .of  vertebrates.  They  seldom 
alight,  but  feed  while  upon  the  wing,  hovering  over  a  flower,  poised 
as  though  resting,  but  continuously  beating  the  air  with  vibrant 
wings,  whose  speed  of  wing-stroke  rivals  that  of  the  insects.  Their 
tiny  eggs  and  nests  are  objects  of  intense  curiosity  among  bird-lovers; 
some  of  the  nests  are  of  the  size  of  a  thimble  and  the  tiny  eggs  are 
like  pearls.  Though  of  miniature  size  the  hummingbirds  are  pugna- 
cious and  full  of  courage,  a  pair  of  them  not  hesitating  to  attack  such 
giant  intruders  as  hawks  and  large  snakes. 

The  swifts  are  less  attractive  than  their  relatives,  the  humming- 
birds, and  are  often  mistaken  for  swallows.  They  have  the  bill  short 
and  broad,  and  the  wide  gape  of  mouth  like  the  goat-suckers. 

The  Colii  (colies)  are  a  small  group  of  somewhat  anomalous  birds, 
that  have  often  been  placed  in  the  order  of  passerine  birds,  but,  on 
account  of  apparent  affinities  with  the  plantain-eaters,  they  are  placed 
in  the  present  position. 

The  Trogones  (trogons)  are  highly  specialized  tropical  birds  of 
comparatively  small  size,  with  long  tail,  short  strong  bill,  and  very 
elaborate  plumage.  The  quezal  (Fig.  158,  A)  is  one  of  the  most 
wonderfully  colored  of  the  trogons,  if  not  of  all  birds.  Its  brilliant 
plumage  of  gold,  metallic  greens  and  blues,  and  its  gracefully  drooping, 
ethereal  plumes  give  it  an  almost  unearthly  beauty. 

The  Pici  (picarian  birds)  include  both  familiar  and  unfamiliar 
types  such  as  the  jacamars  and  puff-birds,  barbets  and  honey-guides, 
toucans,  woodpeckers  and  wrynecks.  Of  these  we  must  be  content  to 
examine  only  the  toucans  and  woodpeckers.  The  toucans  (Fig.  158,  B) 
with  the  possible  exception  of  the  horn-bills,  have  the  most  remark- 
ably specialized  bill  known.  As  Stejneger  says,  "  The  first  thing  which 
strikes  the  observer,  when  looking  at  one  of  the  large  Toucans,  is 
the  enormous  size  of  the  bill.  It  is  not  only  as  long  as  the  bird  itself> 


AVES  309 

but  it  does  not  lack  much  of  equaling  the  body  in  bulk;  and  the  ob- 
server will  most  likely  make  the  remark  that  such  an  enormous  bill 
must  be  very  heavy.  The  fact  is,  however,  that  the  bill  is  extremely 
light  in  comparison  with  its  size,  being  very  thin  and  filled  with  light, 
cellular  bony  tissue."  It  is  not  clear  of  what  value  such  an  enormous 
bill  can  be  to  the  bird,  for  none  of  its  activities  appear  to  be  connected 
with  this  great  structure.  In  all  probablity  this  great  bill  is  an  ex- 
ample of  an  overspecialized  structure,  much  like  the  enormous  horns 
of  the  extinct  Irish  elk,  which  are  believed  to  have  finally  caused  the 
extinction  of  the  species. 

The  woodpeckers  and  sapsuckers  are  among  the  most  familiar 
of  our  native  birds,  and  they  are  especially  known  for  their  habit  of 
riddling  the  bark  and  wood  of  trees  in  their  search  for  insects  and 
larvae,  and  for  their  noisy  drumming  while  engaged  in  this  task.  The 
finest  of  the  woodpeckers  is  the  great  ivory  billed  woodpecker,  which 
has  a  length  of  about  twenty  inches. 

THE  PASSERINE  BIRDS  (PASSERIFORMES) 

This  order,  consisting  largely  of  perching  birds,  is  for  the  Neornithes 
what  the  order  Acanthopterygii  is  for  the  Teleostomi;  the  largest, 
most  varied,  most  distinctively  modern  order  of  the  sub-class.  Over 
five  thousand  species,  or  nearly  half  of  all  known  species  of  birds,  are 
included  within  this  single  order.  The  list  of  families,  thirty-six  in 
number,  is  too  long  to  recite,  but  the  reader  may  get  an  idea  of  the 
scope  and  variety  of  the  order  from  the  following  list  of  representative 
types: — broad-bills,  wagtails,  rock-wrens,  king-birds,  oven-birds,  ant- 
birds,  lyre-birds,  larks,  pipits,  fork-tails,  thrushes,  robins,  warblers, 
gnatcatchers,  mocking-birds,  water-ousels,  wrens,  tits,  swallows  (Fig. 
159,  A),  martins,  wax-wings,  shrikes,  nut-hatches,  greenlets,  titmice, 
orioles  (Fig.  159,  D),  birds  of  paradise,  crows,  ravens,  magpies, 
starlings,  honey-eaters,  sun-birds,  flower-pickers,  creepers,  quit-quits, 
tanagers,  weaver-birds,  finches,  starlings,  buntings,  etc. 

In  general,  it  may  be  said  that  the  passerine  birds  are  of  small  or 
moderate  size,  of  conservative  or  generalized  proportions,  and  without 
exaggerations  of  bill  or  feet.  Some  of  them,  however,  have  developed 
a  wealth  of  plumage  elaborations,  which  is  taken  to  be  one  of  the 
criteria  of  racial  senescence.  Garrod  and  Forbes  subdivide  the  pas- 
serine birds  into  two  sub-orders :  the  Desmodactyli,  in  which  the  hallux 
or  hind  toe  is  weak  and  the  front  toes  are  more  or  less  united;  and  the 


310  VERTEBRATE  ZOOLOGY 

Eleutherodactyli,  in  which  the  hallux  is  the  strongest  toe  and  the  other 
toes  are  free. 

The  Desmodactyli  are  the  broad-bills,  a  single  unfamiliar  type 
native  to  oriental  countries.  They  do  not  differ  outwardly  from  the 
general  standard  of  passerine  birds,  and  are  of  interest  principally  to 
the  systematists. 

The  Eleutherodactyli,  or  free-toed  Passeriformes,  comprise  all 
of  the  remaining  members  of  the  order,  and  cannot  receive  the  pro- 
portionate amount  of  attention  in  the  present  volume  that  their  im- 
portance deserves.  For  particulars  as  to  the  families  of  passerine 
birds  and  the  habits  of  the  numerous  genera  and  species,  the  reader 
is  referred  to  the  many  good  treatises  on  birds.  We  shall  merely  call 
attention  to  a  few  of  the  most  conspicuous  types. 

The  birds-of -paradise  (Fig.  159,  C)  are  without  question  the 
most  elaborately  plumaged  members  of  the  order,  and  constitute  a 
striking  exception  to  the  general  rule,  that  passerine  birds  have  con- 
servative plumage.  The  only  birds  of  other  orders  that  compare 
with  the  birds-of-paradise  in  brilliancy  are  the  long-tailed  trogons  or 
quezals  and  the  hummingbirds,  and  none  of  these  types  have  such 
elaborate  feather  structure.  On  the  whole,  then,  these  birds  may  be 
said  to  cap  the  climax  in  the  evolution  of  plumage  specialization. 
The  great  bird-of-paradise  is  perhaps  the  most  beautiful  of  the  numer- 
ous species.  Apart  from  the  striking  color  scheme,  the  most  remark- 
able specializations  consist  of  a  pair  of  dense  tufts  of  delicate,  droop- 
ing plumes,  that  vary  from  two  to  three  feet  in  length,  arching  upward 
and  then  falling  downward  in  a  veritable  cascade  of  glistening  light. 
Anatomically  speaking,  this  marvelously  handsome  creature  is  no 
more  nor  less  than  "a  glorified  crow/'  for  when  plucked  he  is  seen  to 
be  as  plain  and  common  a  bird  as  is  his  black  cousin. 

The  lyre-birds  (Fig.  159,  B)  of  South  Australia  rival  the  birds- 
of-paradise  in  elaborate  structure  of  plumage,  but  are  not  at  all  bril- 
liant. They  are  moderately  large  birds,  about  two  and  a  half  feet 
long,  with  rather  long  neck,  and  with  fowl-like  head  and  beak.  Their 
only  claim  to  beauty  consists  of  the  remarkable  lyre-like  tail;  the 
sides  of  the  "lyre"  consist  of  two  large  strong  feathers,  that  curve 
outward  from  their  base,  then  curve  inward,  and  again  outward,  at 
the  ends,  in  most  graceful  lines.  Two  middle  feathers,  almost  as 
graceful  as  the  frame-feathers,  cross  each  other  and  droop  out  beyond 
the  outer  feathers;  while  the  remaining  feathers  are  long,  slender  and 


FIG.  159.— Types  of  Passerine  Birds,  showing  generalized  and  specialized 
forms.  A,  Swallow,  Hirudo  rustica;  B,  Lyre-Bird,  Menura  superba;  C,  Lesser 
Bird  of  Paradise,  Paradisea  minor;  D,  Baltimore  Oriole,  Icterus  Baltimore;  E, 
House-sparrow,  Passer  domesticus.  (AH  redrawn;  C,  after  Knowlton,  the  rest  after 
Evans.) 


312  VERTEBRATE  ZOOLOGY 

comparatively  straight,  and  simulate  the  strings  of  the  lyre.  In 
color  these  birds  are  for  the  most  part  of  a  soft  brown,  and  they  are 
not  conspicuous  in  their  natural  haunts. 

The  sparrows  (Fig.  159,  B)  are  usually  placed  last  in  the  systems 
of  classification  because  they  are  believed  to  be  the  most  modern 
type.  They  are  the  most  numerous  and  the  most  familiar  of  all  birds. 
They  are  usually  small  inconspicuously  colored  birds,  characterized 
by  strong,  hard,  conical  bill,  compact  form  and  comparatively  short 
body,  tail,  and  wings.  Possibly  the  most  significant  event  in  the 
history  of  modern  birddom  was  the  invasion  of  North  America  by 
the  English  Sparrow  in  1852.  It  first  landed  in  Brooklyn  and  spread 
from  there  over  the  North  Eastern  Atlantic  States.  In  a  half  century 
it  had  spread  over  a  large  part  of  the  continent  and  is  now  the  most 
numerous  bird  species  in  the  world.  The  English  sparrow  is  a  mod- 
ernist among  birds  and  leads  us  to  discuss  the  probable  future  of  the 
bird  tribe. 

.THE  FUTURE  OF  BIRDS 

It  will  have  been  noted  that  most  of  the  orders  of  birds  have  some 
very  generalized  types  and  some  highly  specialized  types.  One  could 
select  from  nearly  every  order  a  representative  that  is  conservatively 
proportioned  and  has  simple  bill  and  generalized  feet.  In  each  order 
we  also  find  certain  types  with  exaggerated  proportions,  overspecial- 
ized  bill  or  feet  and  highly  colored  or  elaborate  plumage.  If  we  may 
rely  on  the  uniformity  of  nature,  we  may  expect  the  events  of  the 
past  to  repeat  themselves,  and  if  they  do,  these  specialized  birds 
will  become  still  more  senescent  and,  unable  to  reverse  the  course  of 
specialization,  become  extinct;  it  is  all  well  enough  to  be  handsome 
and  brilliant  of  plumage  or  unduly  long  of  leg  or  large  of  bill,  but 
perhaps  the  birds  thus  endowed  will  pay  for  it  in  contributing  to 
the  prehistoric  fauna  of  the  next  geologic  age;  while  the  sparrow  and 
his  ilk  will  still  dispute  with  other  dominant  races  the  domains  of 
earth  and  tree  and  air.  It  is  as  much  as  a  bird's  life  is  worth  now-a- 
days  to  have  beautiful  or  elaborate  plumage,  for  primitive  man  must 
have  its  plumes  for  the  adornment  of  his  primitive  mate;  and  he 
gets  what  the  mate  desires  whether  he  has  to  hunt  the  trackless  for- 
ests for  it,  or  merely  pays  an  exorbitant  milliner's  bill;  a  type  of  bill 
quite  unknown  among  birds.  Safety  for  the  bird  of  to-day  lies  in 
homeliness  of  aspect,  adaptability  as  to  environment  and  food,  and 


AVES  313 

a  goodly  share  of  pugnacity  and  resistance  to  hardship.  Let  the 
modern  birds  consider  the  sparrow  and  his  ways.  He  is  plain  and 
homely,  eats  anything,  lives  anywhere,  builds  his  nests  in  strange  and 
unfamiliar  places,  using  new  and  untried  materials.  He  can  whip 
anything  his  own  size  in  feathers,  but  does  not  needlessly  pick  a 
quarrel,  and  he  can  put  up  with  either  cold  or  heat,  drought  or  flood; 
they  all  look  alike  to  him.  Doubtless  in  the  distant  future  he  will 
dispute  for  the  supremacy  of  the  earth  with  the  mouse,  the  ant,  and 
super-man. 

Man  owes  much  to  the  passerine  birds.  They  give  to  him  who 
has  a  naturalistic  bent  a  keener  zest  for  woodland  life.  Vast  numbers 
of  people  find  their  lives  enriched  by  the  study  of  the  haunts  and 
varied  activities  of  the  birds.  As  destroyers  of  harmful  insects  the 
passerine  birds  are  of  inestimable  value  to  mankind.  It  is  therefore 
of  the  utmost  importance  that  all  agencies  organized  for  the  preven- 
tion of  slaughter  of  the  song-birds  and  other  passerine  birds,  should 
receive  the  united  support  of  every  zoologist  and  lover  of  nature. 
Organizations  such  as  Audubon  Societies  and  the  various  Sportsman's 
Clubs  are  doing  much  to  spread  propaganda  favoring  bird  protection. 
The  writer  of  this  volume  would  like  to  go  on  record  as  unreservedly 
urging  the  support  of  all  agencies  designed  for  bringing  about  the 
enforcement  of  laws  forbidding  the  cruel  and  senseless  slaughter  of 
migrant  passerine  birds. 

MIGRATION  OF  BIRDS 

"The  desire  to  migrate,"  says  Seebohm,  "is  a  hereditary  impulse, 
to  which  the  descendants  of  migratory  birds  are  subject — a  force 
almost,  if  not  quite  as  irresistible  as  the  hereditary  impulse  to  breed 
in  the  spring."  Migrations  follow  more  or  less  direct  paths  between 
winter  homes  and  breeding  quarters.  Most  birds  breed  in  the  north 
and  winter  in  the  south,  Migration  paths  follow  coast  lines,  as  a 
rule,  and  such  locations  as  islands,  capes,  inlets  and  other  good  land- 
marks are  favorite  stopping  places.  Frequently  the  same  birds  stop 
at  the  same  places  several  years  in  succession. 

Birds  have  keen  powers  of  orientation,  and  a  strong  homing  in- 
stinct. This  is  not,  as  some  appear  to  believe,  due  to  a  sixth  sense, 
but  to  a  highly  developed  place  memory,  or  ability  to  recognize 
after  a  lapse  of  time  elements  in  a  landscape  that  have  been  observed 
one  or  more  times  before.  If  a  bird  is  taken  to  an  entirely  new  region 


314  VERTEBRATE  ZOOLOGY 

and  released  it  has  great  difficulty  in  orienting  itself  and  only  suc- 
ceeds in  getting  home  if  by  chance  it  happens  to  discover  a  familiar 
landmark.  Young  birds  are  much  less  capable  of  homing  than  are 
older  birds,  and  need  to  follow  a  leader  until  they  become  familiar 
with  the  route.  Some  birds  migrate  in  flocks  of  great  size,  others  in 
small  numbers  or  even  in  pairs.  The  speed  attained  by  migrating 
birds  may  be  as  high  as  a  hundred  miles  an  hour,  but  the  majority 
of  them  scarcely  attain  half  that  speed.  Even  at  the  rate  of  fifty 
miles  an  hour  birds  have  been  known  to  travel  a  distance  of  nearly 
two  thousand  miles  in  two  days;  for  they  take  little  rest  while  mi- 
grating, and  are  often  entirely  exhausted  when  they  reach  their 
destination.  It  is  during  the  migrating  season  that  ignorant  and 
lawless  people  take  advantage  of  the  large  numbers  of  fatigued  birds 
and  shoot  them  in  vast  numbers,  displaying  in  so  doing  a  lack  of 
sportsmanship  truly  lamentable. 

GEOGRAPHIC  DISTRIBUTION  OF  BIRDS 

Birds  of  good  flying  powers  are  as  nearly  cosmopolitan  as  any 
animal  could  be;  the  albatross  and  the  petrels  range  the  oceans  from 
one  extreme  to  the  other.  Birds  of  moderate  flying  powers  may  be 
limited  to  one  continent;  many  birds,  for  example,  are  confined  to 
North  America,  while  others  breed  in  North  America  and  winter  in 
Central  or  South  America.  Flightless  birds  are  often  confined  to 
single  islands,  such  as  New  Guinea  or  New  Zealand. 

One  should  carefully  distinguish  between  migration  and  distribu- 
tion when  dealing  with  birds;  for  when  we  say  that  a  given  species 
breeds  in  Canada  and  winters  in  Central  America  we  do  not  mean 
that  its  area  of  distribution  covers  all  of  the  intervening  territory, 
for  a  large  part  of  this  territory  is  not  even  passed  over  by  the  species 
in  question  during  the  migration  flights. 

DEVELOPMENT  OF  BIRDS 

The  classic  type  for  the  study  of  avian  embryology  is  the  common 
fowl.  Usually  the  study  of  the  development  of  the  chick  constitutes 
a  major  part  of  college  courses  in  vertebrate  embryology;  hence  only 
a  brief  outline  of  bird  development  need  be  given  here.  There  is  so 
little  difference  between  the  development  of  the  reptile  and  the  bird 
that  the  following  sketch  will  serve  to  illustrate  the  salient  features 
of  the  embryology  of  the  Sauropsida  in  general. 


AVES 


315 


The  "egg"  of  the  bird  (Fig.  160)  is  a  large  and  complex  structure, 
consisting  of  the  ovum  proper,  the  albuminous  layers,  shell  mem- 
branes and  shell.  The  ovum,  or  what  is  usually  referred  to  as  the 
yolk,  is  a  single  food-gorged  cell  inclosed  within  a  vitelline  membrane 
and  with  a  single  nucleus  or  germinal  vesicle.  The  active  protoplasm 
of  the  ovum  is  largely  aggregated  in  a  small  region  situated  at  the 
animal  pole  of  the  cell,  called  the  germinal  spot,  where  lies  the  nucleus. 


ML. 


FIG.  160. — Diagram  of  hen's  egg  to  show  envelopes,  and  general  relations  of 
parts.  A.  C,  air  chamber;  Alb,  albumen;  Bl,  blastoderm;  Chal,  Chalaza;  /.  S.  M, 
inner  layer  of  shell  membrane;  L,  latebra;  NL,  neck  of  latebra;  N.  P,  nucleus  of 
Pander;  O.  S.  M,  outer  shell  membrane;  p.  v.  s,  perivitelline  space;  s,  shell;  B.  M, 
vitelline  membrane;  W.  Y,  white  yolk;  Y.  Y,  yellow  yolk.  (From  Lillie's  "  Develop- 
ment of  the  Chick  "  [Henry  Holt  and  Company].) 

This  small  mass  of  hyaline  protoplasm  is  continuous  with  a  thin 
sheath  of  protoplasm  that  surrounds  and  incloses  the  entire  yolk 
mass  and  to  a  certain  extent  permeates  the  body  of  the  yolk. 

Immediately  surrounding  the  ovum  is  a  thick  viscous  layer  of  al- 
bumen that  is  swathed  about  the  ovum  and  prolonged  on  opposite 
sides  into  twisted  ropes,  called  chalazce,  that  fasten  the  ovum  to  the 
shell  membranes  and  suspend  it  in  such  a  way  that  it  cannot  come 
in  contact  with  the  shell.  Between  the  chalazal  layer  of  albumen  and 


316 


VERTEBRATE  ZOOLOGY 


FIG.  161. — Cleavage  of  hen's  egg.  A,  first  cleavage  furrow  (x  14).  The  egg 
came  from  the  lower  end  of  the  oviduct.  B,  Four-celled  stage  (x  17);  from  the 
uterus;  C,  Ten  central  and  eleven  marginal  cells  (x  about  16);  D,  nine  central  and 
sixteen  marginal  cells  (x  about  16);  E,  late  cleavage  stage  (x  about  22).  (From 
Lillie's  "  Development  of  the  Chick,"  after  Kolliker.) 


AVES 


317 


the  shell  membrane  is  a  second  layer  of  albumen  which  is  quite  fluid 
in  consistency.  Surrounding  the  albumen  is  the  double  parchment- 
like  shell-membrane,  with  an  air-space  between  its  two  layers  at  the 
broad  end  of  the  egg.  The  shell  proper  is  a  rather  complex  structure 
composed  of  calcium  carbonate;  it  is  porous  and  more  or  less  pig- 
ment ed. 

Cleavage  (Fig.  161)  is  strictly  meroblastic,  the  first  cleavage  being 
merely  a  furrow,  and  many  furrows  are  formed  before  any  of  the 
cells  are  furnished  with  floors  or  bottom  partitions  that  cut  them  off 
from  the  underlying  yolk.  Development  proceeds  beyond  the  gas- 


B 


FIG.  162. — Hen's  egg  at  about  the  twenty-sixth  hour  of  incubation,  to  show 
the  zones  of  the  blastoderm  and  the  orientation  of  the  embryo  with  reference  to 
the  axis  of  the  shell.  B,  yolk  of  hen's  egg  incubated  about  50  hours  to  show  the 
extent  of  overgrowth  of  the  blastoderm.  A.  C,  Air  chamber;  a.  p,  area  pellucida: 
a.  v,  area  vasculosa;  a.  v.  e,  area  vitellina  externa;  a.  v.  i,  area  vitellina  interna; 
Y,  uncovered  portion  of  the  yolk.  (From  Lillie,  after  Duval.) 

trula  stage  before  the  egg  is  laid.  A  newly  laid  egg  shows  the  embryo 
as  an  embryonic  disc,  a  small  whitish  spot  at  the  animal  pole,  com- 
posed of  central  transparent  area  (area  pellucida)  bounded  by  an 
opaque  ring  or  germ  wall.  The  pellucid  area  is  two-layered  poste- 
riorly, an  inrolling  of  cells  having  occurred  which  constitutes  the 
primitive  endoderm  and  is  the  equivalent  of  the  archenteron  in- 
vagination  of  the  frog.  The  blastopore  is  crescentic,  as  in  the  frog, 
and  the  primitive  streak  is  formed  by  concrescence  of  the  blastopore. 
The  head  forms  in  front  of  the  primitive  streak,  which  constitutes  the 
axis  of  the  trunk. 


318 


VERTEBRATE  ZOOLOGY 


ce/v/7.  - 


cr/7. 


FIG.  163. — Chick  embryo  of  35  somites,  drawn  as  a  transparent  object,  a.  a. 
1,2,3,  4,  First,  second,  third,  and  fourth  aortic  arches;  Ar,  artery;  A.  V,  vitelline 
artery;  cerv.  Fl,  cervical  flexure;  cr.  Fl,  cranial  flexure;  D.  C,  duct  of  Cuvier; 
Ep,  epiphysis;  On.  V,  ganglion  of  trigeminus;  Isth,  isthmus;  Jug.  ex,  external 
jugular  vein;  Md,  mandibular  arch;  M.  M,  maxillo-mandibular  branch  of  the 
trigeminus;  olf.  P,  olfactory  pit;  Ophth,  ophthalmic  branch  of  trigeminus;  Ot, 
otocyst;  V,  vein;  W.  6,  wing  bud;  V.  C.  p,  posterior  cardinal  vein;  V.  umb,  um- 
bilical vein;  V.  V,  vitelline  vein;  V.  V.  p,  posterior  vitelline  vein.  (From  Lillie's 
"  Development  of  the  Chick.") 


AVES  319 

The  medullary  plate  and  medullary  groove  forms  much  as  in  the 
frog,  beginning  at  the  anterior  end  and  proceeding  to  close  gradually 
from  the  anterior  toward  the  posterior. 

While  the  axial  parts  of  the  embryo  are  differentiating  the  periph- 
eral parts  of  the  blastoderm  continue  to  grow  round  the  yolk,  new 
cells  being  continually  formed  at  the  margin.  Finally  the  whole 
ovum  becomes  covered  with  cells.  A  considerable  part  of  this  sheath 
of  cells  is  destined  to  be  used  only  for  the  formation  of  embryonic 
membranes — amnion,  allantois  and  yolk-sac.  Only  the  parts  near 
the  animal  pole  of  the  egg  are  concerned  in  forming  the  embryo 
proper.  The  embryo  is  gradually  pinched  off  from  the  rest  of  the 
egg  by  means  of  deep  grooves  that  go  so  deep  as  finally  to  leave  only 
a  narrow  yolk-stalk  between  embryo  and  yolk.  An  extensive  vitelline 
circulation  covers  the  yolk  sphere,  and  through  this  means  the  embryo 
maintains  a  nutritive  connection  with  the  yolk. 

Like  all  other  vertebrates  the  young  chick  (Fig.  163)  develops 
four  pharyngeal  clefts  (gill-slits),  only  one  of  which  actually  breaks 
through  to  the  pharynx;  this  is  the  eustachian  tube  of  the  adult.  It 
has  been  commonly  stated  that  the  bird  embryo  never  exhibits  any 
traces  of  gill  filaments  in  these  gill-slits,  but  Boyden  has  recently  de- 
scribed not  only  in  the  chick  but  in  several  reptiles  the  transitory 
appearance  of  tissues,  which  he  believes  are  undeniably  rudimentary 
branchial  filaments. 

Embryonic  Membranes. — The  importance  of  the  amnion  and 
allantois  (Fig.  164)  as  adaptations  for  land  life,  and  their  role  in  the 
evolution  of  the  terrestrial  yertebrates,  have  been  sufficiently  dealt 
with  in  the  chapter  on  reptiles.  In  general  the  mode  of  origin  of 
these  membranes  is  the  same  in  the  bird  as  in  the  reptile.  The  amnion 
begins  as  a  crescentic  fold  of  the  extra-embryonic  blastoderm  in  front 
of  the  head.  This  fold,  which  consists  of  ectoderm  and  mesodern  only, 
grows  backwards,  covering  the  head  like  a  hood  and  continues  to 
spread  over  the  body  until  it  meets  a  smaller,  but  similar  tail  fold  that 
has  been  growing  forward.  The  two  folds  fuse  together  and  com- 
pletely inclose  the  embryo  in  a  sac  lined  with  ectoderm  on  the  in- 
side and  rnesoderm  on  the  outside.  Of  course  an  outer  section  of  the 
fold  is  also  produced,  called  the  chorion,  which  is  lined  with  ectoderm 
on  the  outside  and  with  mesoderm  on  the  inside.  Thus  two  complete 
membranes  shut  off  the  embryo  from  the  albuminous  layers.  The 
inner  layer  of  the  amnion  secretes  an  abundant  watery  fluid  that 


ttmb.cL 


FIG.  164. — Diagram  illustrating  the  development  of  foetal  membranes  in  a 
bird.  A,  early  stage  in  the  formation  of  the  amnion,  sagittal  section;  B,  slightly 
later  stage,  transverse  section;  C,  stage  with  completed  amnion  and  commencing 
allantois;  D,  stage  in  which  the  allantois  has  begun  to  envelop  the  embryo  and 
yolk-sac.  The  ectoderm  is  represented  by  blue,  the  endoderm  by  red,  and  the 
mesoderm  by  gray,  all,  allantois;  all',  the  same  growing  round  the  embryo  and 
yolk-sac;  am,  amnion;  am.  f,  am.f,  amniotic  fold;  an,  anus;  br,  brain;  coel,  coelom; 
coel',  extra-embryonic  ccelom;  h,  heart;  ms.ent,  mesenteron;  mih,  mouth;  nch, 
notochord;  sp.  cd,  spinal  chord;  sr.  mf,  serous  membrane;  umb.  d,  umbilical  duct; 
vl,  m,  vitelline  membrane;  yk,  yolk-sac.  (From  Parker  and  Haswell.) 
320 


AVES 


321 


bathes  the  embryo  throughout  the  entire  embryonic  period  and  pro- 
tects it  from  shocks  and  injuries  due  to  contacts. 

The  allantois  begins  as  an  out-pouching  of  the  hind-gut  not  far 
from  the  yolk-stalk.  It  pushes  outward  as  a  thin-walled  sac,  lined 
with  endoderm  on  the  inside  and  with  mesoderm  on  the  outside, 
grows  out  between  the  amnion  and  chorion,  and  expands  into  a  large 
umbrella-shaped  body  until  it  fills  the  entire  extra-embryonic  coslom, 
or  space  between  the  chorion  and  amnion.  Thus  the  amnion  is  cov- 
ered with  the  distal  part  of  the  al- 
lantois and  the  latter  is  covered  with 
chorion.  In  the  later  stages  these 
three  membranes  fuse  together  in  a 
number  of  places  into  a  single  com- 
pound membrane.  The  allantois  be- 
comes richly  vascular  on  its  outer 
surface  and  acts  as  an  embry- 
onic lung,  getting  oxygen  through 
the  porous  shell  membranes  and 
shell. 

The  yolk-sac  is  at  first  nearly  the 
entire  egg,  but  as  development  pro- 
gresses it  diminishes  in  size  as  the 
yolk  substance  is  assimilated  by  the 
embryo;  until  finally  the  tiny  sac 
that  remains  is  drawn  into  the  body  development  of  head,  long  tail  wing 

J    and  leg    nearly   identical,      (trom 
cavity  of  the  chick  through  the  um-   LiiHe,  after  Keibel  and  Abraham.) 

bilicus,  and  the  latter  closes. 

Changes  in  Body  Form  During  Development. — It  is  of  interest 
to  note  that  the  tail  of  the  chick  of  four  or  five  days'  incubation  is 
comparatively  long  and  slender,  much  like  that  of  a  lizard  at  an 
equivalent  stage  of  development.  Also  the  fore  and  hind  limbs  are 
much  alike  at  that  period,  as  is  shown  in  the  illustration  (Fig.  165). 
It  is  only  after  about  ten  days  of  incubation  (Fig.  166)  that 
the  tail  becomes  foreshortened  into  the  typical  avian  tail  and 
the  fore  limbs  take  on  the  characteristic  features  of  wings.  As  in 
most  other  vertebrate  embryos,  the  head  is  relatively  enormous  as 
compared  with  the  body  during  a  large  part  of  the  embryonic  period, 
and  it  is  only  in  the  last  stages  of  incubation  that  the  body  becomes 
larger  than  the  head.  The  feather  rudiments  appear  about  the  sixth 


FIG.  165. — Chick  embryo  at  five 
days'  incubation,  showing  precocious 


322 


VERTEBRATE  ZOOLOGY 


day  and  are  at  first  mere  papillae  protruding  from  the  skin.  At 
hatching  the  chick  is  completely  covered  with  down-feathers,  which 
are  the  forerunners  of  the  definitive  feathers  and  gradually  give  place 


FIG.  166. — Chick  embryo  at  10  days  and  2  hours,  showing  differentiated  wing 
and  legs,  shortened  tail  and  feather  papillse.  (From  Lillie,  after  Keibel  and 
Abraham.) 

to  the  latter.  On  about  the  seventeenth  day  the  amniotic  fluid  begins 
to  disappear  and  on  the  twentieth  day  it  is  gone.  On  the  same  day 
the  chick,  by  means  of  a  sharp  little  egg-tooth  on  the  point  of  the  bill, 
pecks  a  hole  in  the  shell  and  begins  to  breathe  with  its  lungs.  The 


AVES  323 

allantois  then  gradually  shrivels  up  and  its  circulation  is  cut  off.  On 
the  twenty-first  day  the  chick  bursts  the  shell  and  emerges.  It  is 
quite  a  capable  youngster  at  hatching,  for  it  can  walk,  and  see,  and 
within  a  few  minutes  begins  to  peck  at  the  ground.  I  This  is  quite  in 
contrast  with  the  situation  in  many  other  birds,  w)hose  young  are 
hatched  in  a  naked,  blind  and  entirely  helpless  condition.  As  a  rule, 
birds  that  nest  on  the  ground  have  precocious  young  at  hatching  and 
are  called  Prcecoces  or  Nidifugce;  while  birds  that  nest  in  trees  or  in 
other  safer  retreats  have  helpless  young  and  are  called  Altrices  or 
Nidicolce.  Intermediate  conditions  are  of  course  found  in  many 
species,  especially  in  those  of  sea-birds,  such  as  petrels  and  gulls, 
whose  young  are  downy  at  hatching,  but  stay  in  the  nest  for  some 
time. 

Nesting  Habits  of  Birds. — Any  adequate  account  of  the  nesting 
habits  of  birds  would  require  a  volume  in  itself,  for  there  are  countless 
different  kinds  of  nests  and  of  materials  used.  The  more  primitive 
nests  appear  to  be  crude  nests  built  on  the  ground,  consisting  of  mere 
hollows  scooped  out  of  the  sand  or  earth  after  the  manner  of  some  of 
the  reptiles.  Some  birds  have  no  nests  at  all  but  merely  lay  the  eggs 
on  rocks;  this  is  probably  not  a  primitive,  but  a  degenerate  habit. 
The  members  of  the  higher  orders  of  birds,  as  a  rule,  make  nests  out 
of  grasses  or  other  materials  that  are  suitable  for  weaving  a  fabric  or 
basket-like  container  for  eggs.  These  nests  are  placed  in  trees,  on 
cliff-sides,  in  hollow  trees,  in  burrows  under  the  ground  or  in  caves. 
Clay  or  mud  nests  are  common,  especially  among  swallows.  Birds 
that  occupy  territory  inhabited  by  man  are  quick  to  adopt  the  va- 
rious materials  that  man  furnishes,  such  as  string,  rags,  paper  and 
other  common  waste.  The  use  made  of  various  man-made  bird 
houses  illustrates  the  fact  that  the  bird  is  decidedly  adaptable  and 
not  stereotyped  in  its  form  of  intelligence. 


CHAPTER  IX 
CLASS  VI.  "  MAMMALIA 

Unfortunately  the  only  vernacular  name  for  the  class  Mammalia 
is  mammals,  but  the  man  of  the  street  does  not  know  what  a  mammal 
is.  He  knows  birds,  reptiles,  fishes  and  has  an  idea  that  a  frog  is  an 
amphibian;  but  he  uses  a  variety  of  words  to  express  his  idea  of  a 
mammal,  none  of  which  seems  to  serve  the  purpose  very  well.  He 
sometimes  uses  the  word  "beast,"  but  this  term  does  not  seem  to 
apply  to  men,  at  least  not  to  all  men,  nor  to  whales;  he  uses  the  word 
"quadruped,"  but  this  term  seems  scarcely  appropriate  to  bipeds, 
whales  or  bats. 

It  has  generally  been  assumed  that  mammals  represent  the  apex 
of  organic  evolution,  or  at  least  that  of  the  chordate  phylum.  It  is, 
however,  not  to  be  granted  as  axiomatic  that  the  mammals  represent 
a  higher  level  of  evolutionary  attainment  than  do  the  birds;  for  the 
birds  are  a  more  recent  evolutionary  product,  are  more  nearly  a 
climax  group  to-day,  and  on  the  whole  represent  a  more  highly  spe- 
cialized condition  than  do  the  mammals.  In  only  one  particular  do 
the  mammals  exhibit  a  distinctly  higher  order  of  specialization  than 
do  the  birds;  namely,  in  brain  specialization,  and  particularly  in 
that  of  the  cerebral  hemispheres.  It  has  also  been  said  that  the 
mammals  surpass  the  birds  in  specialization  of  the  teeth  and  of  the 
feet.  This  is  true  in  a  sense,  though  the  toothless  condition  of  the 
bird  and  the  replacement  of  teeth  by  the  bill  is  really  a  more  highly 
specialized  condition  than  any  in  which  the  teeth  still  persist;  while 
the  wing  represents  an  extreme  specialization  of  the  fore  limbs  more 
radical  than  anything  in  the  mammals,  except  possibly  the  flippers 
of  whales.  It  must  be  admitted,  however,  that  the  bird's  hind  limbs 
are  rather  conservative;  for  the  wing  has  rendered  the  functioning  of 
the  foot  of  secondary  importance.  The  claim  of  the  class  Mammalia 
to  supremacy  in  taxonomic  ranking  rests  almost  entirely  upon  their 
superiority  of  nervous  organization.  Man  as  the  exemplar  of  brain 
specialization  adds  immeasurably  to  the  claim  for  supremacy  of  the 
class  to  which  he  belongs;  for  there  is  no  dispute  as  to  the  supreme 

324 


MAMMALIA  325 

status  of  the  human  mammal.  Without  Man  the  mammals  would 
have  at  best  a  disputed  claim  to  highest  rank  among  the  vertebrates; 
with  Man  included,  the  mammals  reign  supreme. 

The  mammals  are  to-day  as  well  denned  in  their  structural  char- 
acters as  are  the  birds,  although  many  of  them  are  exceedingly  aber- 
rant. No  transitional  type  at  all  on  a  par  with  Archceopteryx  exists 
for  the  mammals,  although  the  monotremes  are  intermediate  in  some 
particulars  between  the  typical  mammals  and  the  reptiles,  and  the 
reptilian  order  of  cynodonts  shows  many  mammalian  traits. 

DISTINGUISHING  CHARACTERS  OF  MAMMALS 

>A.  The  skin  (Fig.  167)  is  more  or  less  clothed  with  hair,  though  in 
some  species  there  are  only  a  few  localized  bristles. 

/I.  A  muscular  diaphragm  forms  a  complete  partition  between  the 
thoracic  and  abdominal  parts  of  the  body  cavity;  it  functions  both 
in  respiration  and  in  parturition. 

^3.  Mammary  glands,  with  or  without  teats,  are  always  present. 

4.  Sebaceous  glands  and  sweat  glands  are  always  present  (Fig.  167). 
*o.  The  red  blood  corpuscles  are  non-nucleate  in  the  definitive  con- 
dition and  are  circular  in  outline,  except  in  the  camels  where  they  are 
ovoid. 

H3.  The  cerebral  hemispheres  are  connected  by  a  heavy  commissure 
of  fibers,  called  the  corpus  callosum  (Fig.  171)  'which  is  rudimentary 
in  monotremes  and  small  in  marsupials. 

-?:  There  is  a  single  aortic  arch,  the  left,  which  curves  over  the  left 
bronchus. 

8.  A  larynx,  or  voice-box,  lies  at  the  upper  end  of  the  trachea. 

9.  An  epiglottis,  a  movable  cartilaginous  plate,  covers  the  glottis. 

10.  Lips  and  cheeks  are  characteristic  of  all  mammals  except 
whales. 

11.  The  mandible  (Fig.  169)  consists  of  but  one  pair  of  bones,  the 
dentaries,  which  are  firmly  fused  in  the  adult;  the  dentary  articulates 
directly  with  the  squamosal. 

12.  There  is  a  chain  of  three  bonelets  in  the  middle  ear  that  con- 
nects the  tympanum  with  the  inner  ear.     These  bonelets  are:  the 
stapes  (believed  to  be  homologous  with  the  columella  of  reptiles), 
the  malleus  (believed  to  be  homologous  with  the  articulare  of  rep- 
tiles), and  the  incus  (believed  to  be  homologous  with  the  quadrate  of 
reptiles). 


326  VERTEBRATE  ZOOLOGY 

13.  There  is  an  external  fleshy  and  cartilaginous  conchus  to  the  ear. 

14.  The  body  of  the  vertebra  is  formed  of  three  pieces,  one  of  which 
makes  the  centrum  and  the  other  two  the  epiphyses. 

15.  Cartilaginous  disks  (intervertebral  disks)  separate  the  centra 
of  the  vertebra  from  one  another. 

16.  The  coracoid,  except  in  the  monotremes,  is  represented  by  a 
mere  vestige  fused  with  the  scapula,  called  the  coracoid  process. 

17.  The  ribs,  except  in  monotremes  and  whales,  are  attached  by 
two  heads,  the  capitulum  and  the  tuberculum. 

18.  There  are  characteristically  seven  cervical  vertebrae.     (In  the 
manatees  there  are  but  6,  and  in  the  sloths  there  may  be  6,  8  or  9.) 

•  ~T9.  With  few  exceptions  (whales,  edentates),  mammals  are  diphyo- 
dont,  i.  e.,  have  two  sets  of  teeth,  a  milk  and  a  permanent  dentition. 

20.  The  teeth  are  (a)  thecodont  (each  imbedded  in  an  alveolar  pit 
in  the  jaw  bone);  and  (b)^heterodont  (differentiated  into  incisors, 
canines  and  molars) ;  exceptionally  homodont  or  absent. 
«^21.  There  are  two  occipital  condyles,  which  are  part  of  the  exoc- 
oipital  bones. 

22.  Except  in  moriotremes  there  is  no  distinct  cloaca. 
^23.  Mammals  are  homothermous  (warm-blooded),  with  a  well- 
developed  temperature-regulating  apparatus. 

/24.  The  heart  is  completely  four  chambered,  with  two  auricles  and 
two  ventricles  and  a  complete  double  circulation. 

25.  With  the  exception  of  monotremes,  the  eggs  are  minute  in  size 
and  the  young  are  born  alive  (viviparous). 

There  are  no  gills  nor  gill_rudiments  at  any  stage  of  development. 
-^27.  An  amnion  and  an  allantois  are  always  present;  but  the  allan- 
tois  is  sometimes  vestigial  or  functionless. 

The  first  19  characters  and  20  (b)  distinguish  the  mammals  from 
all  other  living  vertebrates. 

Numbers  21  and  22  distinguish  the  mammals  from  modern  reptiles 
and  birds  (Sauropsida). 

Numbers  23  and  24  distinguish  the  mammals  from  the  reptiles. 

Number  25  distinguishes  the  mammals  from  the  birds. 

Numbers  26  and  27  distinguish  the  mammals  from  the  Amphibia. 

The  Mammalian  Integument. — Under  this  head  we  shall  discuss 
briefly  hair;  skin  glands;  and  claws,  hoofs  and  nails. 

The  possession  of  hair  is  as  truly  diagnostic  for  mammals  as  are 
feathers  for  birds.    Even  the  apparently  naked,  glossy-skinned  whales 


MAMMALIA 


327 


have  a  few  bristle-like  hairs  on  the  upper  lip.  Sometimes  hairs  may 
be  fused  into  scale-like  or  horn-like  structures  as  in  the  scaly  ant- 
eaters  and  the  rhinoceros.  Again  they  may  be  more  or  less  covered 
or  obscured  as  in  the  armor  of  the  armadillos.  The  hair  arises  from 
a  slight  thickening  of  the  Malpighian  layer  of  the  epidermis,  which 
subsequently  invaginates  so  as  to  form  a  deep  pocket  or  follicle  of  the 
epidermis  and  a  dermal  papilla  pushes  up  into  the  bottom  of  the  in- 
vagination  after  the  manner  of  a  pulp  cavity  in  a  tooth.  Thus  the 
origin  and  development  of 
the  hair  is  totally  different 
from  that  of  a  scale  or  of  a 
feather.  The  hair  is  like 
nothing  else;  it  is  sui 
generis. 

There  are  many  kinds  of 
skin  glands  among  mam- 
mals, but  they  may  all  be 
reduced  to  two  fundamental 
types: sudoriparous  or  sweat 
glands  and  sebaceous  glands. 
Generalized  sweat  and  seba- 
ceous glands  (Fig.  167)  are 
scattered  over  nearly  the 
entire  skin,  while  local  spe- 
cializations of  both  types 


FIG.  167. — Section  of  human  skin.  Co, 
dermis;  D,  sebaceous  glands;  F,  fat  in  der- 
mis;  G,  vessels  in  dermis;  G.  P,  vascular  pa- 
pillae; H,  Hair;  N,  nerves  in  dermis;  N.  P, 
nervous  papillae;  Se,  horny  layer  of  epidermis; 
S.  D,  sweat  gland;  8.  Dl,  duct  of  sweat  gland; 
SM,  Malpighian  layer.  (From  Wiedersheim.) 


occur  in  all  mammals.    The 

mammary     glands     of    the 

monotremes  are  specialized 

sweat   glands,    while    those 

of  the  Eutheria  are  specialized  sebaceous  glands.     A  great  many 

mammals  possess  scent  glands  located  in  various  regions.     These 

serve  a  variety  of  uses,  principal  among  which  are:  to  attract  the 

opposite  sex;  to  enable  gregarious  forms  to  distinguish  their  kind; 

and  for  defensive  purposes,  as  in  the  skunk  and  his  tribe. 

Either  claws,  hoofs,  or  nails  are  present  in  all  mammals  except  the 
whales;  even  in  the  latter  rudiments  of  claws  appear  in  the  fcetus  and 
are  subsequently  lost.  Thes3  three  distinct  types,  and  the  total  ab- 
sence of  any  such  structures,  serve  to  divide  the  mammals  into  four 
great  sections:  the  clawed,  the  nailed,  the  hoofed,  and  the  whales, 


328 


VERTEBRATE  ZOOLOGY 


which  have  no  such  structures.  It  is  probable  that  the  claw  is  the 
most  primitive  type  and  that  the  nail,  the  hoof,  and  the  naked- 
fingered  type  represent  a  phyletic  series  of  specializations.  This 
idea  is  more  fully  discussed  in  connection  with  the  mammalian  orders. 
The  Mammalian  Skull.  —  The  skull  of  the  mammal  (Fig.  168) 
differs  from  that  of  other  vertebrates  in  a  number  of  important  par- 
ticulars. It  is  more  compact  and  contains  fewer  elements  than  that 
of  the  reptile.  The  following  bones  characteristic  of  the  reptilian 
ancestry  have  disappeared  from  the  adult  cranium:  pre-  and  post- 
orbitals,  pre-  and  post-frontals,  basipterygoids,  quadrato-jugals,  and 


s. 


FIG.  168. — Skull  of  mammal  (dog).  C.  occ,  occipital  condyle;  F,  frontal; 
F.  inf,  infra-orbital  foramen;  Jg,  jugal;  Jm,  premaxilla;  L,  lachrymal;  Af,  maxilla; 
M.  aud,  external  auditory  meatus;  Md,  mandible;  N,  nasal;  P,  parietal;  Pal, 
palatine;  Pjt,  zygomatic  process  of  squamosal;  Pt,  "pterygoid";  Sph,  alisphenoid; 
Sq,  squamosal;  Sq.  occ,  supra-occipital;  T,  tympanic.  (From  Wiedersheim.) 

supra-temporals.  In  the  lower  jaw,  the  angulare,  splenial,  and  ar- 
ticulare  are  gone;  but  the  latter  is  believed  to  have  been  drawn  in  to 
form  the  malleus,  one  of  the  ear  bonelets.  The  quadrate  has  also 
been  drawn  in  to  form  the  incus  bonelet. 

Mammalian  Dentition. — The  teeth  of  mammals  (Fig.  169)  are 
attached  only  to  the  dentary,  maxillary,  and  premaxillary  bones. 
They  are  limited  in  number,  rarely  exceeding  fifty-four.  The  in- 
cisors are  generally  simple  in  structure  and  with  a  single  root;  the 
canines,  when  present,  are  also  simple  and  with  a  single  root;  the  re- 
maining teeth  (cheek-teeth)  are  divided  into  premolars  and  molars, 
and  show  a  wide  range  of  complexity  in  structure  and  in  number  of 


MAMMALIA 


329 


roots.    They  range  from  simple  one-cusped  teeth  like  canines  to  those 

with   a  large    number   of   cusps. 

The  primitive  types  of  cheek-teeth 

are  provided  with  conical  tuber- 
cles, and  are  known  as  bunodont;  a 

more   highly   specialized   type  of 

tooth  has  the  tubercles  connected 

by  ridges,  and  is  known  as  lopho- 

dont.     There  are  usually  two  sets 

of  teeth,  a  milk  dentition  and  a 

permanent  dentition,  a  condition 

known  as  diphyodont  in  contradis-   .   FIG.  169.-Teeth  of  dog.^  second 
,.  .  incisor;  c,  canine;  pm  1,  pm  4,  nrst  and 

tmction    to    the    condition    char-   fourth   premolars;  m  1,  first    molar. 

acteristic  of  the  lower  vertebrates,    (From  Hegner,  after  Shipley  and  Mac- 

which  have  but  one  set  of  teeth  Bride-) 

(monophyodoni) .  In  the  lowest  mam- 
mals there  is  no  second  dentition,  or 
only  a  partial  replacement  of  the  first 
by  the  second  set.  Many  mammals 
also  have  degenerate  dentition,  involv- 
ing an  entire  loss  of  teeth  or  merely  a 
loss  of  incisors,  or  canines,  or  some  of 
the  molars. 

A  typical  tooth  (Fig.  170)  consists  of 
three  kinds  of  tissue:  enamel,  dentine, 
and  cement.  The  enamel  is  derived 
from  the  epithelium  of  the  mouth  cav- 
ity and  is  therefore  ectodermal;  the 
other  constituents  are  dermal  in  origin. 
The  teeth  arise  as  tooth-germs  quite 
independent  of  the  jaws  and  later  be- 
come imbedded  in  sockets  of  the 
latter.  The  dental  epithelium  is  at 

FIG.  170. — Diagrammatic  section  of  various  forms  of  teeth.  I,  incisor  or  tusk 
of  elephant  with  pulp  cavity  open  at  base.  II,  human  incisor,  during  develop- 
ment, with  pulp  cavity  open  at  base.  Ill,  completely  formed  human  incisor, 
opening  of  pulp  cavity  small.  IV,  human  molar  with  broad  crown  and  two  roots. 
V,  molar  of  ox,  enamel  deeply  folded  and  depressions  filled  with  cement.  Enamel, 
black;  pulp,  white;  dentine,  horizontal  lines;  cement,  dots.  (From  Hegner,  after 
Flower  and  Lydekker.) 


330 


VERTEBRATE  ZOOLOGY 


first  invaginated  as  a  continuous  fold,  covering  the  jaw  from  end  to 
end;  this  fold  is  known  as  the  enamel  organ.  At  intervals  thicken- 
ings occur  at  the  bottom  of  the  groove,  each  of  which  becomes  bell- 


C.7V? 


Fit  If 


c.rs. 


FIG.  171. — Brain  of  rabbit,  especially  to  show  corpus  callosum.  (Nat.  size.) 
In  A  the  left  parencephalon  is  dissected  down  to  the  level  of  the  corpus  callosum : 
on  the  right  the  lateral  ventricle  is  exposed.  In  B  the  cerebral  hemispheres  are 
dissected  a  little  below  the  level* of  the  anterior  genu  of  the  corpus  callosum;  only 
the  frontal  lobe  of  the  left  hemisphere  is  retained;  of  the  right  a  portion  of  the 
temporal  lobe  also  is  left;  the  velum  interpositum  and  pineal  body  are  removed, 
as  well  as  a  greater  part  of  the  body  of  the  fornix,  and  the  whole  of  the  left  pos- 
terior pillar;  the  cerebellum  is  removed  with  the  exception  of  a  part  of  the  right 
lateral  lobe.  a.  co,  anterior  commissure;  a.fo,  anterior  pillar  of  fornix;  a.  pn,  an- 
terior peduncles  of  cerebellum;  b.fo,  body  of  fornix;  eft1,  superior  vermis  of  cer- 
ebellum; c.  62,  its  lateral  lobe;  c.gn,  corpus  geniculatum;  c.  h,  cerebral  hemi- 
sphere; c.  ph,  choroid  plexus;  cp.  cl,  corpus  callosum;  cp.  s,  corpus  striatum; 
c.  rs,  corpus  restiforme;  d.  p,  dorsal  pyramid;  ft,  flocculus;  hp.  m,  hippocampus; 
m.  co,  middle  commissure;  ol1,  anterior,  and  ol2,  posterior  lobes  of  corpus  quad- 
rigemina;  o.  th,  optic  thalamus;  o.  tr,  optic  tract;  p.  co,  posterior  commissure; 
p.fo,  posterior  pillar  of  fornix;  pn,  pineal  body;  pd.  pn,  peduncle  of  pineal  body; 
p.  pn,  posterior  peduncle  of  cerebellum;  p.  va,  fibres  of  pons  Varolii  forming  mid- 
dle peduncles  of  cerebellum;  sp.  lu,  septum  lucidum;  st.  I,  stria  longitudinalis; 
is,  taenia  semicircularis;  v.  vn,  valve  of  Vieussens;  v\  third  ventricle;  v4,  fourth 
ventricle.  (From  Parker  and  Haswell.) 

shaped,  with  a  dermal  papilla  in  the  hollow  of  the  bell.  The  top  of 
the  bell  continues  to  grow  out  as  the  tooth  and  soon  ruptures  the 
gum  and  protrudes  as  a  naked  cusp.  Sometimes  the  enamel  be- 


MAMMALIA  331 

comes  folded  into  deep  pockets  and  gives  to  the  tooth  a  complex 
cross-section,  as  in  the  ungulates.  The  tooth  remains  hollow,  a  pulp 
cavity  remaining  in  its  center,  which  contains  blood-vessels,  nerves, 
and  connective  tissue.  The  dentine  is  merely  a  fine  quality  of  bone 
and  has  the  histological  structure  of  the  latter. 

The  Mammalian  Brain. — Although  the  brains  of  certain  archaic 
mammals  were  not  much  more  highly  developed  than  those  of  some 
of  the  reptiles,  those  of  modern  mammals,  especially  those  of  the  more 
highly  specialized  groups,  show  marked  advances  over  the  brains  of 
other  vertebrates.  The  mammal  brain  (Fig.  172)  is  relatively  large, 
but  the  cerebral  hemispheres  show  more  increase  than  do  other  parts. 
These  hemispheres  are  connected  by  an  elaborate  system  of  commis- 
sures, which  serve  to  correlate  the  two  and  to  make  them  act  as  one 
organ;  the  corpus  callosum  is  the  most  important  of  these  commis- 
sures and  it  reaches  a  large  size  in  the  highest  mammals.  The  surface 
of  the  cerebrum,  in  all  but  the  more  primitive  mammals,  is  much  in- 
folded into  a  system  of  convolutions,  which  greatly  increase  the  sur- 
face without  unduly  increasing  its  bulk.  It  is  not  strictly  true  that 
the  degree  of  complexity  of  the  cerebral  convolutions  is  an  index  of 
the  grade  of  intelligence;  for  the  elephant  has  the  most  elaborately 
convoluted  cerebrum,  but  is  hardly  as  intelligent  as  many  other 
mammals  with  less  convolutions.  The  optic  lobes  are  four  in  num- 
ber, but  in  size  they  are  small.  The  cerebellum  is  scarcely  as  elaborate 
as  in  the  birds,  though  better  developed  than  in  any  reptile. 

Urogenital  Systems  of  Mammals. — The  kidneys  are  compact  in 
form  and  are  of  the  metanephros  type.  They  are  usually  asym- 
metrical in  position,  one  lying  more  anteriorly  than  the  other.  The 
ureters  lead  directly  to  the  urinary  bladder,  which  is  formed  out  of 
the  remains  of  the  allantois. 

The  ovaries  are  always  paired;  never  single  as  in  the  bird.  They 
are  very  small  in  size,  since  they  produce  minute  eggs  with  little  or 
no  yolk.  This  small  size  of  ovaries  and  eggs  is  in  correlation  with 
the  habit  of  uterine  gestation.  The  paired  oviducts  enlarge  into 
paired  uteri,  which  in  some  groups  unite  into  a  single  median 
uterus. 

The  testes  lie  at  first  in  the  body  cavity,  as  in  reptiles,  and  occupy 
positions  homologous  with  those  of  the  ovaries.  In  most  mam- 
mals (monotremes,  whales,  elephants,  armadillos,  and  a  few  others 
excepted)  the  testes  descend  through  the  gubernaculum  into  the 


332 


VERTEBRATE  ZOOLOGY 


scrotum.  The  penis  of  the  male  mammal  is  homologous  with  the 
clitoris  of  the  female  and  is  a  structure  quite  unique  among  verte- 
brates. 


m 


~\rH 


jrrr 


m 


FIG.  172. — Brain  of  dog.  A,  dorsal,  B, ^ventral;  C,  lateral  aspect.  B.  ol,  olfac- 
tory bulb;  Cr.  ce,  crura  cerebri;  Fi.  p,  great  horizontal  fissure;  HH,  HH',  lateral 
lobes  of  cerebellum;  Hyp,  hyophysis;  Med,  spinal  cord;  NH,  medulla  oblongata; 
Po,  pons  Varolii;  VH,  cerebral  hemispheres;  Wu,  middle  lobe  (vermis)  of  cerebel- 
lum; I-XII,  cranial  or  cerebral  nerves.  (From  Wiedersheim.) 

THE  ORIGIN  OF  MAMMALS 

It  has  long  been  held  that  the  mammals  are  descended  from  the 
reptiles,  a  theory  based  on  the  fact  that  the  monotremes,  primitive 
egg-laying  mammals,  have  many  distinctly  reptilian  characters.  If 
mammals  descended  from  any  other  vertebrate  class  they  must  have 


MAMMALIA  333 

come  from  either  Amphibia  or  birds.  The  latter  possibility  is  out 
of  the  question,  for  the  birds  are  more  recent  than  the  mammals. 
There  is,  however,  some  ground  for  the  idea  that  the  mammals  may 
have  been  derived  directly  from  the  Amphibia. 

The  Theory  of  Amphibian  Ancestry  of  the  Mammals. — This 
theory  was  advanced  by  Huxley  and  gained  considerable  vogue  until 
recent  discoveries  eliminated  it  from  the  field.  Huxley's  argument 
in  brief  was  as  follows:  The  presence  of  two  occipital  condyles  sep- 
arates the  mammals  from  the  reptiles,  and  unites  them  with  the 
Amphibia.  The  mammals  retain  the  left  aortic  arch  and  lose  the 
right,  while  birds  retain  the  right  arch  and  lose  the  left.  Reptiles 
show  a  tendency  to  reduce  the  left  arch,  which  does  not  look  to- 
ward a  mammalian  condition,  and  therefore  discredits  the  reptile 
ancestry  idea. 

This  theory  is  based  on  the  supposition  that  the  condyles  and 
aortic  arches  of  modern  reptiles  are  primitive  and  were  the  same  in 
the  early  reptiles  as  they  are  to-day.  This  is  a  fallacy,  however, 
for  some  of  the  early  reptiles,  notably  the  cynodonts,  had  two  con- 
dyles like  the  Amphibia.  It  is  also  quite  possible  that  the  early 
reptilio-mammal  stock  had  a  reversed  symmetry  of  the  aortic 
arches.  Although-  still  advocated  by  some  modern  writers,  the 
theory  of  amphibian  ancestry  of  the  mammals  confidently  may 
be  set  aside. 

Palaeontological  Evidence  of  the  Origin  of  the  Mammals. — Within 
comparatively  recent  years  the  fossil  evidences  of  mammalian  de- 
scent have  been  vastly  strengthened  by  the  discovery  in  Triassic 
rocks  of  South  Africa  of  a  large  collection  of  remains  of  a  group  of 
extinct  reptiles  known  as  Cynodontia  (dog-toothed),  that  have  al- 
ready been  dealt  with  in  the  chapter  on  reptiles.  There  were  many 
types  of  cynodonts,  some  of  which  exhibit  mammalian  tendencies  of 
one  sort,  others  of  another.  Certain  authorities  claim  that  all  of  the 
distinctions  between  reptiles  and  mammals,  based  on  bony  structures, 
are  transgressed  by  one  or  more  groups  of  cynodonts,  some  groups 
transgressing  with  regard  to  a  majority  of  distinctions,  other  groups 
with  regard  to  one  or  a  very  few.  These  creatures  were  obviously 
reptiles  of  a  rather  generalized  type  in  most  respects,  but  they  were 
evidently  making  some  of  the  same  experiments  that  the  ancestral 
mammals  must  have  made  in  order  to  arrive  at  the  present  mamma- 
lian status.  Whether  or  not  these  cynodonts  were  the  actual  an- 


334  VERTEBRATE  ZOOLOGY 

cestors  of  the  first  mammals  it  is  impossible  to  say,  but  there  is  noth- 
ing inherently  improbable  about  such  a  theory. 

The  cynodonts  (Fig.  173)  were  mammal-like  in  a  number  of  ways: 
a,  they  had  a  well-defined  heterodont  dentition,  with  incisors,  canines 
and  molars;  b,  they  had  two  condyles;  c,  the  lower  jaw  was  composed 
primarily  of  the  dentaries,  but  there  were  sometimes  small  vestigial 

angulare,  articulare,  and  other  rep- 
tilian bones;  d,  the  quadrate  was 
often  greatly  reduced   and   must 
have  been  functionless  as  a  con- 
nection between  the  mandible  and 
Art          the  skull.    These  and  many  minor 
features  of  the  skull  and  limb  skel- 
FIG.  173.— Skull  of  cynodont  reptile    etons    were  modified   in   a  mam- 
Nythosaurus     larvalus,     Trias,     South          ,.         ..         .  . 

Africa.  Note  mammal-like  tooth  dif-  mahan  direction,  but  no  single  spe- 
ferentiation,  but  complex  reptilian  cies  of  cynodont  approached  very 
lower  jaw.  Ang,  angulare;  Art,  articu-  closely  a  true  mammalian  condi- 
lare;  Dent,  dentary;  Ju,  jugal;  L,  lach-  .  J  .,  ,  ,  f  ,  ,  . 
rymal;  MX,  maxillary;  Na,  nasal;  Pa,  tlon-  Possibly  the  future  has  in  store 
parietal;  Pmx,  premaxillary;  PoO,  post-  for  the  palaeontologists  the  disco v- 
orbital;  Pr.  F,  prefrontal;  S.  Ang,  ery  of  the  real  ancestral  mammal, 
surangulare;  sq.  squamosal.  (  Irom  __ 

Lull,  after  Broom.)  Now>  the   cynodonts  belong  to 

the  sub-class  Synapsida  and  the 

order  Therapsida,  which  Williston  places  very  low  in  the  series  of 
reptilian  orders,  far  below  the  Ichthyosauria,  Squamata,  Rhyncho- 
cephalia,  Crocodilia,  Dinosauria,  and  Pterosauria.  The  only  orders 
of  lower  rank  are  Cotylosauria,  Chelonia,  and  Theromorpha.  It  seems 
highly  probable  that  the  Therapsida  were  derived  from  an  early  very 
generalized  group  of  Permo-Carboniferous  theromorphs,  probably  the 
pelecosaurs,  of  which  Varanosaurus  appears  to  be  the  most  general- 
ized representative.  This  type  was  a  long  lizard-like  reptile  with 
very  generalized  proportions  and  with  the  maxillary  teeth  somewhat 
more  prominent  than  the  others. 

In  addition  to  the  mammal-like  skull  characters  referred  to  above, 
these  South  African  cynodonts  had  modifications  of  the  limbs  (Fig, 
123,  E)  that  appear  to  have  had  to  do  with  rapid  locomotion,  char- 
acters  that  might  well  have  served  to  introduce  the  habit  of  migration 
and  thus  to  have  given  these  reptiles  an  advantage  over  their  more 
sluggish  relatives.  Migrations  would  have  a  tendency  to  increase 
the  powers  of  observation  and  in  turn  to  have  served  to  accelerate 


MAMMALIA  335 

brain  development.  The  habit  of  living  in  regions  with  a  changeable 
temperature  would  doubtless  be  associated  with  the  development  of 
various  temperature-regulating  mechanisms  that  are  to-day  associated 
with  what  we  call  warm-bloodedness.  Probably  the  gradual  develop- 
ment of  the  homothermous  condition  paralleled  the  gradual  separa- 
tion of  the  right  and  left  ventricles  and  the  resultant  complete  sep- 
aration of  the  arterial  and  venous  blood.  One  of  the  consequences  of 
a  higher  temperature  must  have  been  a  heightened  nervous  efficiency, 
for  it  is  well  known  that  nervous  tissues  tend  to  develop  more  ef- 
fectively at  relatively  high  temperatures.  Furthermore,  the  habit 
of  uterine  gestation  would  increase  the  effectiveness  of  the  higher 
temperatures  at  the  very  time  when  the  organism  is  most  responsive; 
for,  as  has  been  experimentally  demonstrated,  the  early  stages  of 
development  are  crucial  in  determining  the  character  of  the  nervous 
system.  (In  support  of  this  view  it  may  be  said  that  the  least  highly 
differentiated  brain  among  modern  mammals  is  that  of  the  mono- 
tremes  in  which  there  is  a  less  effective  temperature-regulating  mech- 
anism and  in  which  a  constant  developmental  temperature  is  im- 
possible because  the  eggs  are  allowed  to  cool  periodically  while  the 
mother  is  absent  in  search  of  food.  The  most  highly  differentiated 
brain,  moreover,  is  found  in  Man,  who  has  an  exceptionally  prolonged 
period  of  uterine  gestation,  and  who  has  learned  the  uses  of  clothing 
and  artificial  heat  as  aids  in  maintaining  a  constant  high  body  tem- 
perature, especially  in  the  young  infant  prior  to  the  development  of 
its  homo  thermic  mechanism;  for  the  human  infant  is  for  some  time 
after  birth  practically  cold-blooded  in  the  sense  that  it  is  unable  to 
maintain  a  constant  temperature.) 

Time,  Place  and  Environment  of  the  Pro-mammals. — A  knowl- 
edge of  the  period  when  the  pro-mammals  lived  should  give  a  clew  as 
to  the  probable  causes  of  the  development  of  mammalian  characters 
in  some  reptilian  group.  The  place  of  origin  of  the  first  mammalian 
experiments  appears  to  have  been  South  Africa,  and  the  time  early 
Permian.  The  eminent  palaeontologist  and  palaeogeographer,  Schu- 
chert,  says:  "  The  evidence  is  now  unmistakable  that  early  in  Permian 
times  all  of  the  lands  of  the  southern  hemisphere  were  under  the  in- 
fluence of  a  glacial  climate  as  severe  as  the  polar  one  of  recent  times, 
and  that,  like  the  latter,  the  Permian  one  also  had  warmer  interglacial 
periods,  for  coal  beds  occur  associated  with  glacial  deposits  in  Aus- 
tralia, South  Africa,  and  Brazil."  Now  the  cynodonts,  which  we 


336  VERTEBRATE  ZOOLOGY 

have  dealt  with  as  the  group  of  reptiles  showing  the  most  pronounced 
mammalian  tendencies,  were  not  Permian  animals  at  all,  but  lived  in 
Triassic  times,  a  million  or  so  years  later.  How  then  could  they  have 
given  rise  to  the  mammals?  The  answer  is  that  they  themselves 
probably  did  not  produce  the  mammals,  but  that  they  and  the  mam- 
mals were  both  derived  from  a  common  ancestral  stock  that  lived  in 
the  Permian.  The  mammals  represented  an  offshoot  of  this  ancestral 
stock  that  went  the  entire  course  in  developing  mammalian  charac- 
ters, while  the  cynodonts  represent  a  number  of  partially  successful 
experiments  that  fell  short  in  various  respects  of  full  mammalian  de- 
velopment. Some  day  it  is  hoped  that  the  true  ancestral  mammals 
will  be  found  in  rocks  deposited  not  later  than  the  Middle  Triassic 
and  not  earlier  than  the  Lower  Permian. 

MESOZOIC  MAMMALS 

The  first  actual  relics  of  mammals  proper  appear  in  the  Triassic 
contemporaneously  with  the  cynodont  reptiles.  It  is  believed  that 
the  beginning  of  mammalian  evolution  took  place  about  ten  million 
years  ago  and  that  the  first  mammals  were  very  small  creatures  about 
the  size  of  rats  or  mice.  Osborn  believes  them  to  have  been  arboreal 
forms,  probably  insectivorous,  and  obliged  to  lead  a  furave  nocturnal 
life.  In  the  daytime  they  hid  among  the  trees  and  thickets  and  at 
night  ventured  forth  in  search  of  prey.  It  may  well  have  been  crea- 
tures of  this  sort  that  were  partially  or  largely  responsible  for  the 
slaughter  of  eggs  and  young  of  the  great  Mesozoic  reptiles,  for  they 
must  have  lived  together  during  this  period.  Osborn  thinks  that 
these  small  furry  creatures  probably  resembled  the  modern  tree- 
shrews,  such  as  Tupaia  (Fig.  186,  B),  a  species  which  he  believes  to 
be  the  most  nearly  prototypic  of  the  modern  mammals. 

In  the  study  of  mammalian  evolution  particular  attention  must  be 
paid  to  the  two  mechanisms  whose  contact  with  the  environment  is 
the  most  intimate;  the  teeth  and  the  feet.  For  the  evolution  of  the 
mammalian  orders  and  families  is  primarily  one  of  foot  and  tooth 
specialization;  hence  these  two  characters  are  of  fundamental  im- 
portance in  the  classification  of  the  Mammalia. 

The  teeth  of  reptiles,  except  the  cynodonts,  are  simple  conical 
bodies,  with  little  or  no  regional  differentiation.  The  cynodonts,  as 
we  have  seen,  had  incisors,  canines  and  primitive  molars;  the  mam- 
mals have  carried  out  this  differentiation  much  further.  The  molars 


MAMMALIA  337 

tend  to  become  tuberculate  (bunodont)  in  some  groups,  and  flatten 
out  into  broad,  crushing  teeth  (lophodont)  in  others.  In  some  orders 
the  incisors  are  modified  into  great  chisel-edged  gnawing  teeth;  in 
other  groups  they  become  vestigial  or  are  totally  lost  in  the  adult. 
The  canines  on  the  whole  are  the  most  conservative  of  the  teeth,  tend- 
ing to  retain  their  conical  shape;  but  in  some  groups  they  have  be- 
come specialized  into  tusks  of  various  kinds,  and  in  other  groups  they 
are  vestigial  or  absent. 

The  feet  of  primitive  reptiles  are  typical  five-fingered  feet  with 
claws.  From  this  type  of  foot,  it  will  be  recalled,  the  reptiles  under- 
went an  extensive  adaptive  specialization.  Hoofed  feet  were  de- 
veloped in  the  heavy  herbivorous  forms  and  long  raptorial  claws  de- 
veloped among  the  carnivorous  groups.  A  similar  adaptive  radiation 
in  foot  structure  occurred  among  the  mammals.  It  is  probable  that 
the  first  mammals  were  unguiculate  (clawed),  a  condition  very  similar 
to  the  generalized  ancestral  foot.  From  this  type  were  developed 
the  various  three-fingered,  two-fingered,  and  one-fingered  hoofed 
types,  the  curving-clawed  preda- 
ceous  types,  the  flat-nailed  types  of 
the  primates,  and  all  of  the  other 
specialized  types  of  foot  structure. 

The  earliest  mammalian  remains      ,-, 
/-n-      IT/IX  •  >     e  >         •        i  *IG-  *•*• — Jaw  of  primitive  mam- 

,^lg.  17$)  consist  Ot  two  jaw  bones   m^   Dromatherium  sylvestre,   Trias, 

found  in  the  coal  beds  of  North  N.  Carolina;  twice  natural  size. 
Carolina,  a  Triassic  deposit.  The  (From  Lull,  after  Osborn.) 
creatures  to  which  these  jaws  belonged,  whether  they  were  true  mam- 
mals or  not,  must  therefore  have  been  contemporaneous  with  the 
South  African  cynodonts.  Except  for  the  fact  that  the  jaw  was  a 
one-piece  jaw  consisting  only  of  the  paired  dentary  bones,  it  was 
more  like  those  of  the  cynodonts  than  like  those  of  modern  mam- 
mals. The  molar  teeth  were  very  generalized  in  that  their  num- 
ber of  tubercles  was  indefinite  and  the  incisors  were  only  slightly 
flattened. 

A  somewhat  later  group  of  primitive  mammals,  known  as  Tricono- 
donta,  is  represented  by  a  few  fragmentary  remains  (Fig.  175)  found 
in  Jurassic  rocks.  These  mammals  had  teeth  more  perfect  in  form 
than  those  just  described,  the  molars  being  trituberculate  with  the 
cusps  knife-edged  and  arranged  in  a  single  row  like  the  teeth  of  a  saw. 
Another  group  of  early  mammals  of  Lower  Cretaceous  and  Jurassic 


338  VERTEBRATE  ZOOLOGY 

times  had  teeth  in  which  the  tubercles  are  arranged  in  a  triangular 
or  trigon  group,  with  the  main  cusp  on  the  inner  edge  of  the  tooth. 
These  Trituberculata,  as  they  are  called,  probably  had  insectivorous 
habits  and  may  have  been  the  direct  ancestors  of  the  insectivores  of 


FIG.    175. — Jaw   of  Triconodont   mammal,    Triconodon  ferox,    Comanchian, 
Wyoming.    Three  times  natural  size.    (From  Lull,  after  Marsh.) 

to-day.  Still  another  early  group,  the  Allotheria  (Fig.  176),  found  as 
early  as  the  Jurassic,  but  lasting  over  into  the  Cenozoic,  had  multi- 
tuberculate  molars  and  rodent-like  incisors.  The  premolars  were  in 
some  cases  much  like  the  primitive  cutting  teeth  of  the  Trituberculata. 

All  of  these  mammalian  relics  indicate 
that  the  mammals  made  a  very  modest 
and  tentative  start  in  the  Mesozoic.    If 
one  may  judge  by  the  teeth, 'they  had 
already    undergone   a    limited    adaptive 
radiation  into  insectivorous,  carnivorous, 
FIG.  176.— Skull  of  multi-  ancj  gnawing  types,  which  foreshadowed 
tuber culate    mammal     (allo-    .,  ,.  «  ,.,       ,    ,  .. 

there)  PtOodus  gracilis,  Pal-  the  mammalian  groups  of  like  habits  to- 
seocene.  (Ft.  Union) ;  Wyom-  day.  The  reason  for  their  inconspicuous- 
ing.  About  natural  size.  ness  during  the  Mesozoic  is  not  far  to  seek, 
(From  Lull,  after  Gidley.)  r  ,  ,. e  ,  ,  .,  ,  , 

for  they  lived  when  the  reptiles  had  pre- 
empted all  of  the  important  life  ranges.  Their  very  inconspicuous- 
ness  was  their  salvation  and  gave  them  a  chance  to  live  through  a 
trying  period  and  to  await  the  dawning  of  their  great  opportunity; 
this  came  toward  the  end  of  the  Cretaceous,  when  the  reptilian 
dynasties  waned  and  extinction  overtook  all  of  the  highly  spe- 
cialized dominant  types.  Perhaps,  as  has  already  been  suggested, 
these  small,  blood-thirsty  mammals  played  an  important  role  in 
hastening  the  decline  of  the  reptiles  by  preying  on  their  eggs 
and  young.  One  may  picture  the  Mesozoic  drama  in  the  words 
of  Lull,  if  we  bring  "before  tlie  mind's  eye  broad  vistas  of  low- 


MAMMALIA  339 

lying  well-watered  woodland  with  ever  alert  furry  forms  taking 
such  refuge  as  the  trees  and  shrubbery  or  occasional  hiding  holes 
could  offer,  in  the  midst  of  stalking  terrors  such  as  the  world 
never  saw  before  or  since.  That  the  mammals  managed  to  maintain 
themselves  is  not  surprising,  for  there  is  a  teeming  horde  of  small 
mammalian  folk  in  the  tiger-haunted  jungles  of  India  to-day;  and 
that  they  did  not  dispute  with  the  dinosaurs  the  realms  of  greater 
opportunity  is  but  a  logical  assumption." 

In  conclusion  it  may  be  said  of  the  Mesozoic  mammals  that  they 
made  less  headway  in  the  Mesozoic  than  did  the  reptiles  in  the 
Palaeozoic;  for  they  were  all  quite  generalized  in  structure  and  of 
small  size.  There  is  evidence  that  the  first  mammals  arose  very 
soon  after  the  reptiles  became  well  established  in  the  Permian.  If 
this  is  so,  we  see  that  all  of  the  vertebrate  classes  except  the  birds 
had  their  origin  in  Palaeozoic  times. 

CENOZOIC  MAMMALS 

The  Cenozoic  has  been  called  the  Age  of  Mammals,  just  as  the 
Mesozoic  is  called  the  Age  of  Reptiles.  The  same  great  climatic  or 
geological  conditions  that  are  assumed  to  have  led  to  the  extinction 
of  the  exuberant  reptilian  dynasties  that  flourished  during  the  Meso- 
zoic may  be  also  given  the  credit  for  affording  the  mammals  their 
first  opportunity  to  " secure  for  themselves  a  place  in  the  sun." 
After  a  lurking  life  in  the  shades  and  shadows  they  were  able  to 
emerge  into  the  open  and  to  invade  the  vast  fields  of  opportunity 
vacated  by  the  fallen  races  of  reptiles.  The  small,  warm-blooded 
mammalian  races  repeopled  the  wastes,  gaining  the  upper  hand  over 
the  few  reptilian  groups  that  remained,  such  as  the  lizards,  snakes 
and  turtles;  these  in  turn  took  up  the  furtive  life  that  the  mammals 
left  behind.  The  mammals  had  been  under  pressure  during  the  en- 
tire Mesozoic,  and  when  the  pressure  was  removed  they  expanded 
marvelously. 

The  Archaic  Mammals  of  the  Cenozoic. — The  mammals  of  the 
early  periods  of  mammalian  deployment  during  the  Tertiary  are 
usually  called  archaic  mammals.  They  differ  from  modern  mammals 
in  the  following  particulars:  1,  Their  feet  were  conservative,  showing 
little  advance  upon  reptilian  conditions;  2,  their  molar  teeth  were 
very  little  differentiated  for  the  various  feeding  habits;  3,  the  brain, 
especially  the  part  which  is  the  main  seat  of  intelligence  (the  cere- 


340  VERTEBRATE  ZOOLOGY 

brum)  was  small  in  proportion  to  the  size  of  the  body  and  was  reptile- 
like  in  many  respects. 

These  archaic  mammals  went  the  ways  of  the  Mesozoic  reptiles  to 
a  considerable  extent,  in  that  they  became  large,  robust,  vegetative 
mechanisms  with  low  intelligence  and  little  adaptability.    Like  the 
dinosaurs  they  vanished  completely  from  the  face  of  the  earth  and 
left  few  descendants.    Only  those  of  small  size  and  with  comparatively 
unspecialized  structures  survived  to  become  the  ancestors  of  our 
modern  mammalian  faunas.    Osborn  sums  up  the  situation  as  follows: 
"Nature  deals  in  transitions  rather  than  in  sharp  lines.    We 
can  not  circumscribe  the  archaic  mammals  sharply,  nor  be  sure 
as  yet  that  some  of  them  did  not  give  direct  descent  to  certain 
of  the  modernized  mammals.     Yet  the  animals  of  the  basal 
Eocene  of  both  Europe  and  North  America  are  altogether  of  a 
very  ancient  type;  they  exhibit  many  primitive  characters,  such 
as  extremely  small  brains,  simple  triangular  teeth,  five  digits 
on  the  hands  and  feet,  and  prevailing  plant igradism.    They  are 
to  be  collectively  regarded  as  the  first  grand  attempts  of  nature 
to  establish  insectivorous,  carnivorous  and  herbivorous  groups, 
or  unguiculate  (clawed  forms)  and  ungulates  (hoofed  forms). 
The  ancestors  or  centres  of  these  adaptive  radiations  date  back 
into  the  Age  of  Reptiles.    At  the  beginning  of  the  Eocene  we 
find  the  lines  all  separated  from  each  other,  but  not  as  yet  very 
highly  specialized.     The  specialization  and  divergence  of  these 
archaic  mammals  continue  through  the  Eocene  period  and  reach 
a  climax  near  the  top,  although  many  branches  of  this  archaic 
stock  become  extinct  in  the  Lower  Eocene.    The  orders  which 
may  be  provisionally  placed  in  this  archaic  group  are  the  follow- 
ing: 

"Marsupialia. 

Multituberculata,  Plagiaulacidse. 
Placentalia. 

Insectivora.    Insectivores  not  yet  positively  identified  in 

the  basal  Eocene. 

Tseniodonta.    Edentates  with  enamel  teeth. 
Creodonta.    Archaic  families  of  carnivores. 
Condylarthra.    Primitive  light  limbed  cursorial  ungulates. 
Amblypoda.      Archaic,    typically   heavy    limbed,    slow- 
moving  ungulates. 


MAMMALIA 


341 


"This  group  is  full  of  analogies,  but  is  without  ancestral  affin- 
ities to  the  higher  placentals  and  marsupials.  There  are  forms 
imitating  in  one  or 
more  features  the  A 

modern  Tasmanian 
'wolf       (Thylacy- 
nus),     the     bears, 
cats,  hysenas,   civ- 
ets, and  rodents  of 
to-day,     but      no 
true    members    of 
the      orders     Pri- 
mates,      Rodents, 
Carnivora,     Peris- 
sidactyla,      Artio- 
dactyla  have  been 
discovered." 
The  outstanding 
groups  of  archaic  mam- 
mals are  the  Creodonta, 
the    Condylarthra,   and 
the  Amblypoda.    These 
three     claim    our    at- 
tention. 

The  Creodonta 
(flesh-toothed)  differ 
from  the  Condylarthra 
in  having  the  skull  and 
tooth  characters  of  car- 
nivores, and  in  having 
claw-like  rather  than  FIG.  177.— Creodonts.  A,  Tritemnodon,  a  primi- 
hoof-like  terminal  pha-  tive  hysenodont,  Middle  Eocene,  North  America, 
langes.  The  most  evi- 


dent     difference      be- 


(After  Scott).  B,  Hycenodon,  the  last  survivor  of 
the  archaic  carnivores,  Lower  Oligocene,  North 
America  and  Old  World.  (After  Osborn).  C,  the 

tween     the      creodonts   dog-like  Dromocyon,  Middle  Eocene,  North  America. 

«nH  mnrWn    PnrnivnrP*    (After    Osborn)-      D>    Patriofelis,    Middle    Eocene, 

and  modern  carnivores  North  America     (A11  from  Lull|  after  Osborn.) 
is   in  the    capacity    of 

the   brain-case;   for,    like   all    archaic   mammals,    they   had   small 
reptile-like  brains.     The  teeth  of  the  creodonts  are  also  less  spe- 


342 


VERTEBRATE  ZOOLOGY 


cialized  for  rending  of  flesh  than  are  those  of  the  true  carnivores. 
Of  the  six  families  of  creodonts  recognized  by  palaeontologists,  all 
but  one  became  extinct  before  the  dawn  of  the  era  of  modernized 
mammals.  Perhaps  the  best-known  of  the  creodonts  is  the  genus 
Dromocyon,  which  is  shown  in  the  illustration  (Fig.  177,  C).  It  is 
interesting  to  note  that  there  were  bear-like,  dog-like,  otter-like,  cat- 
like, and  hyaena-like  creodonts. 

The  Condylarthra  (knuckle-jointed)  were  archaic  ungulates  and 
differed  from  the  creodonta  mainly  in  adaptations  for  herbivorous 
diet.  In  form  they  closely  resembled  the  creodonts,  for  both  were 
rather  generalized  in  structure.  The  most  interesting  of  the  con- 
dylarthrans  is  Phenacodus  (Fig.  178),  a  form  that  for  a  long  time 

was  believed  to  be  the  five- 
toed  ancestor  of  the  horse, 
but  is  now  known  to  be 
both  too  specialized  in  some 
respects  and  too  late  in  its 
appearance  to  have  the 
honor  of  playing  this  role. 
Only  a  few  species  of  con- 
FIG.  178. — Cursorial  archaic  mammal,  con-  dylarthrans  are  known  and 
dylarth,  Phenacodus  primcevus.  Lower  Eocene,  ,,  -,  •  , 

North  America.    (Fr£m  Lull,  after  Osborn.)        these     are     SrouPed     mto 

two  families.     They  range 

from  the  size  of  a  fox  to  that  of  a  large  sheep.  They  had  rather 
tusk-like,  but  small,  canines  and  low-crowned  grinding  teeth  of 
archaic  pattern.  The  skull  was  long  and  low  with  a  small  brain 
case.  The  feet  were  five-fingered  and  of  a  primitive  plantigrade  form, 
with  small  hoofs.  The  genera  that  have  been  studied  could  not  have 
given  rise  to  modern  ungulates,  but  the  real  ancestors  of  the  ungulates 
must  have  been  relatives  of  the  condylarths. 

The  Amblypoda  (blunt-footed)  were  short-footed  ungulate-like 
mammals,  some  of  which  attained  a  huge  size,  almost  comparable 
with  that  of  the  elephants,  but  reminding  one  more  of  the  rhinoceros 
or  hippopotamus  types.  There  were  four  families  of  amblypods 
that  differed  considerably  among  themselves. 

The  genus  Coryphodon  (Fig.  179)  is  one  of  the  best  known  ambly- 
pods. It  was  probably  a  swamp-dweller,  nearly  as  large  as  an  ox, 
but  much  more  thick-set  and  massive.  The  limbs  were  short  and 
powerful,  and  the  feet  had  spreading  toes  well  adapted  for  swamp 


MAMMALIA 


343 


navigation.    The  skull  was  long  and  flat,  and  without  horn?.    The 
canine  teeth  were  tusk-like  and  were  probably  used  for  tearing  out 

the  succulent  roots  of  water 
plants.  Everything  seems  to 
indicate  that  it  was  a  very 
sluggish  and  stupid  creature. 
Dinoceras  (Fig.  180)  rep- 
resents another  family  of  am- 
blypods  and  appears  to  have 
been  an  end-product  of  a 
long  line  of  specializations. 
It  stood  about  seven  feet 
in  height,  had  very  heavy, 
elephantine  limbs  and  a  mas- 
sive body.  The  head  was 
armed  with  two  heavy  horns  and  great  tusks,  which  were  doubtless 
used  as  a  defense  against  the  creodonts,  the  only  contemporary  ani- 
mals that  could  have  attacked  a  creature  of  such  proportions. 

The  archaic  mammals  nearly  all  became  extinct  before  the  end  of 
the  Eocene.    The  causes  of  their  extinction  can  only  be  conjectured. 


FIG.  179. — A  swamp-dwelling  amblypod, 
Coryphodon,  Lower  Eocene,  North  America. 
(From  Lull,  after  Osborn.) 


FIG.   180. — Four-horned  amblypod,  Dinoceras,  the  culmination  of    its  race, 
Upper  Eocene,  Wyoming.    (From  Lull,  after  Osborn.) 

In  some  cases  it  seems  probable  that  .over-specialization  combined 
with  racial  old  age  brought  about  extinction;  in  other  cases  it  is  more 
likely  that  the  competition  against  the  on-coming  modernized  mam- 
mals, which  were  more  alert  and  intelligent,  brought  about  their 


344  VERTEBRATE  ZOOLOGY 

gradual  elimination.  Only  a  very  few  of  the  archaic  mammals  sur- 
vived beyond  Eocene  times  and  these  were  rather  generalized  types, 
suitable  to  be  the  ancestors  of  the  modernized  mammals.  There  was 
probably  considerable  emigration  on  the  part  of  certain  of  these  sur- 
viving archaic  types.  It  may  well  be,  for  example,  that  the  marsupials 
of  Australasia  were  the  descendants  of  a  very  early  group  of  multi- 
tuberculate  mammals  that  succeeded  in  reaching  the  Australasian 
peninsula  before  it  was  cut  off  from  the  Asiatic  continent.  The 
cutting  off  of  the  Australasian  land  bodies  must  have  occurred  before 
the  modernized  mammals  reached  that  part  of  the  world,  for  there 
are  in  those  regions  no  true  modernized  mammals. 

ORIGIN  OF  THE  MODERNIZED  MAMMALS 

The  modernized  mammals  include  practically  all  existing  placental 
mammals  and  their  immediate  ancestors,  including:  true  carnivores, 
rodents,  odd-  and  even-toed  ungulates,  elephants,  sirenians  and  whales. 
To  this  list  some  authors  would  add  edentates,  bats,  and  insectivores. 
"In  contrast  with  the  archaic  mammals,"  says  Lull,  "the 
modernized  types  are  all  creatures  of  high  potentiality,  and, 
where  they  became  extinct,  were  rather  the  victims  of  circum- 
stance than  creatures  that  died  because  of  lack  of  adaptability, 
although  certain  groups  seem  to  have  run  a  natural  course  and 
their  extinction  was  heralded  by  evidences  of  racial  senility. 

"As  the  archaic  forms  were  characterized  by  lack  of  progressive 
brain  and  feet  and  teeth,  so  the  modernized  races  were  distin- 
guished by  the  possession  sometimes  of  one  (primates),  some- 
times of  two  (elephants),  again  by  all  three  (horses)  of  these 
destiny  controlling  organs,  but  in  general  the  modernized  animals 
were  progressive,  highly  adaptable  forms." 

Place  of  Origin. — It  is  believed  that  the  modernized  mammals 
originated  in  the  great  Arctic  Continent.  The  reasons  for  this  belief 
are:  first,  there  is  a  striking  resemblance  between  the  first  European 
and  the  first  North  American  modernized  mammals;  second,  palseo- 
geographers  tell  us  that  a  fluctuating  land  bridge  between  the  eastern 
and  western  continents  existed  from  time  to  time,  and  between  times 
was  submerged;  third,  the  climate  of  the  Arctic  regions  was  at  one 
time  warm,  as  is  evidenced  by  the  discovery  of  fossils  of  sub-tropical 
plants  on  the  coast  of  Greenland. 
Time  of  Origin. — All  available  evidence  seems  to  point  to  the 


MAMMALIA  345 

latter  half  of  the  Eocene  period  as  the  time  when  the  modernized 
mammals  arose.    Some  types  evidently  arose  considerably  later. 

Migrations. — The  spread  of  the  modernized  mammals  must  have 
been  southward.  This  must  have  been  so  for  two  reasons:  first,  that 
was  the  only  possible  direction  in  which  a  group  originating  in  the 
north  could  migrate;  and  second,  because  the  increasing  cold  which 
culminated  in  the  first  glacial  epoch,  must  have  driven  the  majority 
of  the  mammals  out  of  the  northern  regions.  A  few  of  the  hardiest 
types  still  find  these  regions  habitable.  The  migration  occurred  in 
several  great  waves,  probably  due  to  the  alternating  periods  of  cold 
and  warm  climate  in  the  north.  The  groups  least  tolerant  of  cold 
probably  migrated  southward  first  and  went  farthest  south;  among 
these  first  migrants  were  probably  the  insectivores  and  primates; 
these  were  probably  followed  by  the  perissodactyls  (horses  and  tapirs) 
and,  somewhat  later,  by  the  true  carnivores,  especially  the  cat-like 
forms.  The  bears  and  rodents  remained  longer  than  the  rest  and 
still  live  well  toward  the  north.  To-day  the  modernized  mammals 
have  a  world-wide  distribution  except  in  the  oceanic  islands,  which 
they  have  no  means  of  reaching. 

MAMMALS  OF  THE  PRESENT 
Brief  Classification 

CLASS  MAMMALIA.     "Beasts,"  " quadrupeds,"  "animals."    Warm- 
blooded, hair-clad  vertebrates  with  mammary  glands. 

Sub-Class  I.  Prototheria. — Egg-laying  mammals. 
Order  1.  Monotremata. 

Family  1.  Ornithorhynchidse. 
"      2.  Echidnidse. 

Sub-Class  II.  Eutheria. — Viviparous  mammals. 

Division  I.  Didelphia  (Metatheria) . — Marsupials. 

Order  1.  Marsupialia.     Mammals  that  usually  carry 
the  young  in  a  pouch;  usually  no  placenta. 
Sub-Order  1.  Polyprotodontia. 
Sub-Order  2.  Diprotodontia. 

Division  II.  Monodelphia  (Placental  mammals).  Young 
never  carried  in  a  pouch;  a  true  placenta, 
which  nourishes  the  unborn  fetus. 


346  VERTEBRATE  ZOOLOGY 

Section  A.  Unguiculata 

Order  1.  Insectivora. 

2.  Dermoptera. 

"     3.  Chiroptera. 

"     4.  Carnivora. 

"     5.  Rodentia. 

"     6.  Edentata  (Xenarthra). 

"     7.  Pholidota. 

"     8.  Tubulidentata. 

Section  B.  Primates 

Order  9.  Primates. 

Sub-Order  1.  Lemuroidea. 
2.  Anthropoidea 

Section  C.  Ungulata 


Order  10.  Artiodactyla. 

"     11.  Perissodactyla. 

"     12.  Proboscidia. 

"     13.  Sirenia. 

"     14.  Hyracoidea. 

Section  D.  Cetacea 

,    Order  15.  Odontoceti. 
"      16.  Mystacoceti. 

This  classification,  which  follows  closely  that  given  by  Osborn  in 
"The  Age  of  Mammals,"  departs  widely  from  traditional  lines.  The 
grouping  of  five  orders  into  the  section  Ungulata  is  decidedly  novel; 
the  separation  of  the  Proboscidia  from  the  Perissidactyla,  and  the 
inclusion  of  the  Sirenia  among  the  ungulates,  are  well  founded; 
the  distribution  of  the  old  group  of  Edentata  among  several  distinct 
orders  will  doubtless  meet  with  general  approval,  for  it  has  long  been 
felt  that  the  old  assemblage  was  artificial.  But  perhaps  the  most 
striking  feature  of  the  classification  is  the  position  assigned  to  the  Pri- 
mates— below  the  ungulates  and  the  cetaceans  instead  of  at  the  apex 
of  the  phyletic  series  where  we  have  been  accustomed  to  place  them. 
This  somewhat  lowly  position  of  the  Primatt  >  is  justified  by  the  fact 


MAMMALIA 


347 


that,  generally  speaking,  they  are  much  less  specialized  than  are 
either  the  ungulates  or  the  whales.  Even  Man,  apart  from  his  re- 
markable brain  and  his  upright  position,  is  a  comparatively  unspe- 
cialized  mammal.  If  this  disrespectful  treatment  of  lordly  Man  shocks 
the  gentle  reader,  let  him  remember  that  several  authorities  have 
already  assigned  to  the  birds  the  distinction  of  being  the  most  highly 
specialized  vertebrate  class;  so  the  edge  is  taken  off  the  contest  for 
first  honors  in  the  second  division. 

'    SUB-CLASS  I.    PROTOTHERIA   (MONOTREMATA,  EGG-LAYING) 

MAMMALS 

The  modern  representatives  of  this  sub-class  are  few,  consisting  of 
but  three  genera  of  strange  beasts  native  to  Australasia.    Some  frag- 


FIG.  181. — Pectoral  arch  and  sternum  of  Ornithorhynchus  anatinus.  c'1  c2,  c3, 
first,  second  and  third  ribs;  cl,  clavicle;  ec,  epicoracoid;  e$l  and  es2,  prosternum 
(episternum)  m.  c,  metacoracoid  (coracoid) ;  ra.  s,  manubrium  sterni;  sc,  scapula; 
st,  sternebra.  (From  Wiedersheim.) 

mentary  remains  of  Multituberculata,  already  discussed  in  the  sec- 
tion dealing  with  the  Mesozoic  mammals,  have  also  been  assigned  by 
some  authorities  to  this  division. 

The  Monotremata  are  mammals  that  lay  large  eggs  with  a  shell, 
abundant  yolk,  and  albumen  (eggs  practically  reptilian  in  character)  ; 
they  have  diffuse  mammary  glands  without  teats;  the  brain  lacks  the 
corpus  caliosum;  the  shoulder  girdle  has  a  large  coracoid  (Fig.  181) 
reaching  to  the  sternum;  an  interclavicle  is  present;  paired  marsupial 


348  VERTEBRATE  ZOOLOGY 

or  epipubic  bones  extend  forward  from  the  pelvis;  the  vertebrae  are 
for  the  most  part  without  epiphyses;  the  ribs  are  one-headed,  the 
tuberculum  being  absent;  the  mammary  glands  are  modified  sweat- 
glands  and  are  not  sebaceous;  there  is  a  shallow  cloaca;  one  group 
(the  Echidnidse)  has  a  temporary  pouch  for  incubating  the  eggs.  The 
oviducts  are  entirely  separate  throughout  and  open  by  two  separate 
genital  pores  into  the  cloaca. 

The  majority  of  these  characters  hark  back  to  a  reptilian  ancestry 
and  are  therefore  to  be  considered  as  primitive.  It  is  not  believed, 
however,  that  the  monotremes,  as  we  know  them,  are  at  all  close  to 
an  ideal  pro  to  ty  pic  mammalian  condition;  but  rather  that  they  are 
the  end-products  of  a  rather  highly  specialized  side  line  of  mammalian 
evolution,  that  came  off  from  some  early  reptilio-mammal  stock  and 
that  has  retained  some  of  the  primitive  characters  of  these  ancestors. 
It  is  not  thought,  therefore,  that  the  monotremes  are  in  any  sense 
ancestral  to  the  Eutheria. 

Family  1.  Echidnidce. — This  family  contains  two  genera,  Echidna 
and  Proechidnd.  Echidna  aculeata  (Fig.  182,  D),  the  "  Australian 
Anteater,"  is  the  best  known  species.  It  is  found  in  New  Guinea, 
Tasmania  and  Australia,  and  several  local  sub-species  are  distin- 
guished. Its  characters  may  be  dealt  with  under  two  categories: 
those  that  are  csenogenetic,  adaptations  for  the  anteating  habit;  and 
those  that  are  palingenetic  or  primitive. 

Echidna  is  a  typical  anteater  in  all  of  its  adaptations.  It  has  a 
heavy  protective  covering  of  quill-like  spines,  with  an  underlying 
layer  of  coarse  hair.  The  snout  is  long  and  tapering,  reminding  one 
rather  strongly  of  a  bird's  bill.  The  tongue  is  extremely  long  and 
extensible  and  is  covered  with  a  sticky  salivary  secretion,  which  holds 
the  ants  when  the  tongue  is  thrust  into  ant  holes.  The  claws  are 
very  long  and  powerful  and  are  used  for  tearing  down  ant-hills  and 
for  making  burrows.  As  in  anteaters  of  other  orders,  teeth  are  lack- 
ing. Two  other  characters  seem  in  no  way  to  relate  Echidna  to  the 
anteating  habit;  these  are  first,  a  rudimentary  tail,  much  like  that  of 
a  bird,  and  second,  a  small  spur  connected  with  a  peculiar  gland  on 
the  heel,  a  structure  whose  function  is  not  well  understood.  Of  some- 
what more  fundamental  importance  are  the  following  characters:  the 
cerebral  hemispheres  are  fairly  large  and  well  convoluted;  there  is  a 
temporary  marsupial  pouch  (Fig.  182,  C),  which  seems  to -have  no 
relation  to  the  marsupium  of  the  marsupials,  but  is  more  nearly 


MAMMALIA 


349 


FIG.  182. — Group  of  Monotremata.  A,  Proechidna  bruijnii;  B,  Proechidna 
nigroaculeata;  C,  Echidna  aculeata;  ventral  aspect  to  show  brood  pouch;  D, 
Echdina  aculeata;  E,  Ornithorynchus  anatinus;  F,  Ornithorhynchus  standing  up 
like  a  penguin;  G,  Ornithorhynchus  female  allowing  young  to  obtain  milky 
secretion  from  the  diffuse  abdominal  mammary  glands.  (All  redrawn,  A,  B,  F, 
G,  after  Brehm;  C,  after  Haacke;  D  and  E,  after  Vogt  and  Specht.) 


350  VERTEBRATE  ZOOLOGY 

homologous  with  a  teat;  the  temperature  of  the  body  is  lower  than 
in  the  higher  mammals,  and  has  a  variation  in  health  of  at  least  15° 
Centigrade,  a  character  which  seems  to  be  intermediate  between  the 
poikilothermous  and  the  homothermous  conditions. 

Proechidna,  a  New  Guinea  species,  differs  from  Echidna  in  the 
following  particulars:  the  toes  en  both  fore  and  hind  feet  are  reduced 
to  three  large  and  two  rudimentary  elements;  the  beak  is  longer  and 
is  curved  downward;  the  back  is  more  arched;  the  external  lobe  of 
the  ear  protrudes  freely  from  the  hair  of  the  head.  The  combination 
of  characters  gives  to  the  Proechidna  a  ridiculous  resemblance  to  a 
miniature  elephant.  Two  species,  P.  bruijnii  (Fig.  182,  A)  and  P. 
nigroaculeata  (Fig.  182,  B),  are  distinguished. 

The  breeding  habits  of  the  Echinidae  are  of  especial  interest. 
The  egg  is  about  half  an  inch  long  and  has  a  leathery  shell  much 
like  that  of  a  tortoise.  Only  one  egg  is  laid  at  a  time  and  it  is  imme- 
diately transferred  by  the  mouth  of  the  mother  to  the  brood  pouch 
(see  Fig.  182,  C),  /where  it  undergoes  a  short  incubation.  When 
ready  to  hatch,  the  shell  is  broken,  as  in  the  bird,  by  means  of  a  shell- 
breaking  tubercle  on  the  end  of  the  snout;  the  mother  then  removes 
the  broken  fragments  of  shell.  The  just-hatched  young  is  in  a  very 
immature  and  helpless  condition  and  lies  quietly  in  the  pouch  for 
some  tune,  merely  able  to  lap  up  the  milky  secretion  that  exudes  from 
the  walls  of  the  pouch.  After  the  young  has  reached  a  considerable 
size  it  is  removed  by  the  mother  from  time  to  time  in  order  to  give 
it  exercise,  but  it  is  put  back  into  the  pouch  to  be  suckled.  There  is 
among  Echidnidse  really  no  need  of  a  nest,  for  the  egg  is  kept  safely 
in  a  pouch.  After  a  time,  however,  the  mother  leaves  the  young  in 
the  burrow  while  she  pursues  her  nocturnal  occupation  of  ant-hunting. 
This  burrow  with  its  enlarged  terminal  chamber  is  a  safe  retreat  for 
the  youngster  when  later  he  ventures  forth  to  learn  the  anteating 
game  for  himself. 

Family  2.  Ornithorhynchidce. — This  family  consists  of  but  the 
single  species  Ornithorhynchus  anatinus  (Fig.  182,  E),  the  Duck-bill 
Platypus,  a  native  of  Southern  Australia  and  Tasmania.  When  the 
first  specimen  of  this  strange  beast  was  exhibited  in  England  it  was 
believed  to  be  a  fake,  on  a  par  with  the  composite  mermaids  then  in 
vogue.  It  was  described  as  a  furry  quadruped  with  the  bill  and  feet 
of  a  duck;  a  very  apt  characterization.  The  animal  is  about  a  foot 
and  a  half  long,  with  a  heavy  coat  of  soft  brown  fur.  The  feet  are 


MAMMALIA 


351 


/FC 


five-toed  and  webbed,  the  webbing  on  the  fore  feet  extending  well 
beyond  the  tips  of  the  toes,  but  that  of  the  hind  feet  being  about  as 
it  is  in  a  water  bird.  Both  feet  are  armed  with  sharp  claws.  The 
beak  is  very  wide  and  flat  and  is  covered  with  soft,  naked  skin  that 
flares  out  at  the  base  into  sensitive  flaps;  this  beak  covering  is  highly 
sensitive  owing  to  the  abundance  of  sense  organs  that  are  scattered 
over  its  surface.  There  are  no  teeth  in  the  adult,  but  instead,  broad, 
horny  plates  line  the  inside  of  the  bill;  these  are  used  for  crushing  the 
shells  of  bivalves  and  water  snails,  which  constitute  its  chief  food. 
The  young  platypus  has  a  set  of  milk  teeth,  all  molars  and  eight  or 
ten  in  number;  these  are  gradually  worn  off  and  then  replaced  by 
plates.  The  eyes  are  small  and  beady;  there  is  no  external  ear  looe; 
the  male  has  a  spur  on  the  heel  like  that  of 
the  Echidnidse,  but  larger  in  size.  The  tail 
is  large  and  dorso-ventrally  flattened;  it  is 
used  as  a  rudder  in  swimming. 
u-The  brainjjf  Ormthorhynchus^'Fig.  183)  is 
the  most_primitive  brain_known  for  a  living 
mammal.  It  is  comparatively  quite  small, 
and  the  cerebral  hemispheres  are  smooth 
and,  like  a  reptile  brain,  entirely  lacking 
in  convolutions.  The  habits  of  this  creature 
are  purely  aquatic,  not  unlike  those  of  a 
muskrat.  It  lives  in  stagnant,  weedy  ponds 
or  streams,  feeding  chiefly  on  mollusks, 
crustaceans,  and  worms  that  are  secured  by 
scooping  up  the  muddy  bottom  with  the 
spoon-like  snout.  Provender  is  stored  in 
capacious  cheek-pockets  and  is  carried  in 
this  way  to  the  burrow,  where  it  is  eaten  at 
leisure.  The  burrow  is  dug  deep  into  the  bank  of  the  stream,  begin- 
ning below  the  water-line  and  sloping  upward  until  at  a  distance  of 
twenty-five  to  fifty  feet  it  terminates  in  a  large,  dry  chamber  with 
top  ventilation.  The  chamber  is  comfortably  lined  with  reeds  and 
rushes. 

Breeding  Habits. — The  eggs  to  the  number  of  two  or  three  are 
laid  in  a  nest  of  grasses,  quite  like  a  simple  bird's  nest.  They  are 
somewhat  smaller  than  those  of  Echidna  and  have  a  rather  hard, 
flexible  shell,  yellowish-white  in  color.  They  are  incubated  while 


s 


FIG.  183.  —  Brain  of 
Orniihorynchus,  dorsal 
view,  natural  size;  cbl,  cer-f 
ebellum;  olf,  olfactory 
bulbs.  Note  lack  of  cere- 
bral convolutions.  (From 
Parker  and  Has  well.)  - 


352  VERTEBRATE  ZOOLOGY 

still  in  the  nest  by  means  of  the  body  heat  of  the  mother;  hence  there 
is  no  brood  pouch.  When  the  young  hatch  they  are  fed  by  a  milky 
secretion  which  exudes  from  the  primitive  abdominal  milk-glands 
that  are  buried  deep  in  the  hair.  The  young  simply  licks  off  the 
drops  of  milk  as  they  drip  from  the  wet  hairs.  When  the  youngsters 
are  older  the  mother  lies  on  her  back  (Fig.  182,  G)  and  the  ludicrous 
little  fellows  climb  on  top  of  her  in  order  to  feed  to  better  advantage. 

SUB-CLASS  II    EUTHERIA  (VIVIPAROUS  MAMMALS) 

Definition. — Mammary  glands  are  of  the  sebaceous  type  and  are 
provided  with  teats;  brain  has  a  corpus  callosum  between  the  cerebral 
hemispheres;  coracoid  is  vestigial  and  does  not  reach  the  sternum; 
there  is  no  interclavicle;  ribs  are  double  headed;  vertebrae  have 
epiphyses;  ovum  is  small;  young  are  born  alive. 

The  group  includes  both  marsupials  and  the  placental  mammals. 
There  is  a  much  closer  resemblance  between  the  marsupials  and  the 
placentals  than  between  the  former  and  the  monotremes;  hence  it 
has  seemed  justifiable  to  group  the  marsupials  and  the  placentals  in 
one  sub-class. 

DIVISION  I.  DIDELPHIA  (METATHERIA)— -MARSUPIALS 

Definition.- — Mammals  with  small  eggs  that  are  usually  provided 
with  a  thin  shell  and  a  thin  layer  of  albumen.  The  oviducts  are  en- 
larged into  a  pair  of  uterine  pouches  which  are  sometimes  fused  for  a 
short  distance.  The  distal  parts  of  the  oviducts  remain  entirely 
separate,  giving  a  double  vagina,  a  character  responsible  for  the  name 
Didelphia.  The  egg  develops  in  the  uterus,  absorbing  nutriment 
through  its  membranes.  In  rare  cases  a  primitive  allantoic  placenta 
is  present.  The  young  is  born  in  a  very  immature  condition  and  is 
placed  by  the  mother  in  the  marsupium  (Fig.  185,  B)  or  brood  pouch 
(not  present  in  all  marsupials),  and  is  fed  from  the  milk  glands  by 
means  of  a  long  tubular  teat  (Fig.  185,  C  and  D)  that  is  thrust  down 
the  throat,  and  to  which  the  young  is  attached  send-permanently 
by  means  of  a  special  larval  mouth  sucker.  The  marsupials  have 
epipubic  bones;  have  rudimentary  corpus  callosum;  a  shallow  cloaca 
is  present  in  at  least  some  species.  The  skull  has  the  following  pe- 
culiarities that  are  useful  in  identifying  fossil  species:  incompletely 
ossified  palate,  jugal  bone  reaching  as  far  as  the  glenoid  cavity; 
teeth  more  numerous  than  is  typical  for  placentals;  molars  generally 


MAMMALIA  353 

four  on  each  side;  usually  but  one  tooth  of  the  milk  set,  the  fourth 
premolar,  is  functional. 

In  general  it  may  be  said  that  the  marsupials  occupy  a  position 
intermediate  between  the  monotremes  and  the  placental  mammals. 
They  have  undergone  an  elaborate  adaptive  radiation,  occupying  in 
their  native  countries  most  of  the  life  zones  that  are  in  other  parts 
of  the  world  occupied  by  the  placentals.  Their  favorite  life  zone  is 
the  arboreal  and  they  seldom  invade  the  aquatic  zones.  There  are 
some  highly  specialized  cursorial  types;  some  sub-terrestrial,  fossorial 
types;  and  some  semi-volant  types.  They  have  produced  several 
giant  forms,  now  extinct,  but  recent  forms  are  small  or  of  moderate 
size. 

While  they  were  at  one  time  numerous  and  fairly  well  distributed 
over  Europe  and  North  America  they  are  now  almost  confined  to 
Australasia.  A  number  of  species  of  opossums  and  the  rat-like  Cceno- 
lestes  belong  to  the  American  continent,  mostly  to  South  America. 
None  are  found  in  Europe,  Asia  or  Africa  to-day. 

It  is  believed  by  some  authorities  that  the  marsupials  spread  to 
Australia  and  South  America  over  the  hypothetical  Antarctic  land 
bridge  and  were  subsequently  cut  off  before  the  placental  mammals 
were  evolved.  They  have  persisted  in  Australia  largely  because  they 
have  escaped  competition  with  the  larger  and  more  capable  modern- 
ized mammals  that  ruled  the  other  continental  bodies.  A  more  care- 
fully considered  theory,  however,  would  derive  the  marsupials  from 
northern  forms  that  migrated  southward  to  escape  the  rigors  of  the 
early  glacial  epochs,  and  reached  Australia  and  South  America  before 
the  onset  of  the  dominant  placentals.  The  cutting  off  of  Australasia 
gave  them  their  best  opportunity  for  adaptive  expansion. 

It  is  not  now  believed  that  the  marsupials  represent  a  stage  in 
the  evolution  of  the  placental  mammals;  rather  it  is  thought  that 
they  represent  the  adaptive  radiation  of  a  primitive  mammalian  stock 
that  arose  far  back  in  the  Mesozoic  (probably  during  the  Jurassic) 
and  has  had  an  evolution  of  its  own,  somewhat  less  successful  and 
slower  than  that  of  the  modernized  groups.  They  show  many  evi- 
dences of  racial  senility  and  some  of  their  supposedly  primitive  fea- 
tures may  well  be  the  products  of  regressive  processes.  The  fact 
that  there  are  only  traces  of  the  milk  dentition,  and  the  occurrence 
in  some  species  of  a  transitory  allantoic  placenta,  have  been  inter- 
preted as  retrograde  conditions  and  as  evidences  in  favor  of  the  idea 


354  VERTEBRATE  ZOOLOGY 

that  at  one  time  the  marsupials  were  more  fully  diphyodont  and  had 
a  true  placental  gestation. 

Regarding  the  marsupials  as  a  single  order,  we  may  divide  them 
into  two  sub-orders:  Polyprotodontia  (many  incisors),  and  Diproto- 
dontia  (two  incisors) .  The  first  group  is  now  believed  to  be  the  more 
primitive  and  the  second  more  highly  specialized  and  somewhat 
senescent. 

SUB-ORDER  I.  POLYPROTODONTIA 

This  group,  which  consists  mainly  of  insectivorous  and  carnivorous 
types,  is  more  primitive  than  are  the  herbivorous  diprotodonts.  The 
polyprotodonts  are  characterized  by  the  possession  of  four  or  five 
incisors  on  each  side  of  the  upper  jaw  and  one  or  two  fewer  in  the 
lower  jaw;  both  canines  and  molars  have  the  typical  carnivorous 
shape.  They  are  confined  to  Australasia,  with  the  exception  of  the 
American  opossums. 

Family  1.  Didelphidce  (the  Opossums). — Of  all  living  marsupials 
the  opossums  appear  to  be  the  most  generalized  in  both  structure 
and  habits.  The  Virginia  opossum  (Fig.  184,  A),  Didelphys  virginiana, 
is  the  only  North  American  member  of  the  family  and  deserves  spe- 
cial mention.  It  is  distinctly  arboreal,  with  a  prehensile  tail  adapted 
for  clinging  to  branches  and  for  use  as  a  hold-fast  by  the  young,  who 
wind  their  tails  about  the  arched  tail  of  the  mother.  The  opossum 
is  omnivorous,  eating  fruit,  insects,  birds,  reptiles,  and  their  eggs. 
There  is  a  distinct  pouch  in  which  the  young  are  suckled  and  carried. 
The  animal  is  nocturnal  in  habit,  sleeping  in  hollow  trees  during  the 
day.  The  death-feigning  instinct  has  received  the  proverbial  de- 
scription " playing  'possum."  Important  genera  of  the  family  are: 
Didelphys,  Marmosa,  Chironectes,  Peramys,  and  Philander;  there  are 
about  twenty-five  species,  all  American.  Marmosa  murina  is  a  tiny 
opossum  about  the  size  of  a  small  rat;  Chironectes  is  an  aquatic  type 
with  webbed  feet  and  about  the  size  of  a  muskrat.  It  is  the  only 
aquatic  marsupial  (Fig.  184,  D). 

Family  2.  Myrmecobiidce  (Banded  Anteaters). — This  small  family 
is  represented  by  a  single  species,  Myrmecobius  fasciatus  (Fig.  184,  B), 
an  animal  about  the  size  of  a  cat,  with  only  slight  specializations  for 
the  anteating  habit.  Its  snout  is  moderately  prolonged ;  its  tongue  is 
very  long  and  extensible  and  is  covered  with  the  customary  sticky 
secretion;  the  tail  is  covered  with  long,  coarse  hair;  the  claws  are  only 


MAMMALIA 


355 


moderately  heavy.    Instead  of  being  toothless  like  the  anteaters  of 
other  orders  they  have  an  unusually  large  number  of  small  teeth. 


G  H 

FIG.  184. — Group  of  Marsupials  (Polyprotodonts).  A,  Virginia  Opossum, 
Didelphys  virginiana;  B,  Banded  Ant-Eater,  Myrmecobius  fascialus;  C,  Native 
Cat,  Dasyurus  viverrinus;  D,  Water  Opossum,  Chironectes  minima;  E,  Marsupial 
Mole,  Notoryctes  typhlops;  F,  Rabbit  Bandicoot,  Peragole  lagotis;  G,  Thylacine 
or  Tasmanian  Wolf,  Thylacinvs  cynocephalus;  H,  Tasmanian  Devii,  Sarcophilus 
vrsinus.  (All  redrawn,  A  after  Vogt  and  Specht,  D,  after  Lydekker;  B,  after 
Flower  and  Lydekker;  E,,  after  Beddard;  others,  after  Brehm.) 


356  VERTEBRATE  ZOOLOGY 

ranging  from  50  to  54.  In  this  respect  and  in  several  others  they 
resemble  the  Mesozoic  marsupials.  Myrmecobius  has  no  pouch. 

Family  3.  Dasyuridce  (Carnivorous  Masuspials). — This  is  a  some- 
what heterogen?ous  family  of  marsupials,  ranging  from  mouse-like 
to  badger-like  types.  They  may  or  may  not  have  a  pouch.  Dasyurus 
viverrinus,  the  "  native  cat "  (Fig.  184,  C)  is  less  cat-like  in  appearance 
than  marten-like.  It  feeds  largely  on  birds  and  their  eggs.  Sar- 
cophilus  ursinus,  the  "Tasmanian  devil"  (Fig.  184,  H),  is  an  animal 
about  the  size  and  shape  of  a  badger.  It  has  the  reputation  of  being 
one  of  the  most  ferocious  of  animals,  with  a  devilish  "yelling  growl." 
Native  Australians  say,  however,  that  it  is  rather  a  slinking  than  an 
openly  pugnacious  creature.  Phascologale  is  a  genus  of  small  animals 
not  unlike  some  of  the  smaller  American  opossums  in  appearance  and 
habits.  Sminthopsis  is  a  genus  of  pouched  mice.  Antechinomys  is  a 
genus  of  jumping  mice,  with  long  ears  and  legs. 

Family  4-  Thylacynidce  (Thylacynes). — This  family  is  represented 
by  the  single  species  Thylacinus  cynocephalus  (Fig.  184,  G),  which 
receives  the  name  of  the  "Tasmanian  wolf."  The  creature  is  less 
like  a  wolf  than  like  some  of  the  smaller  members  of  the  Cat  family, 
but  the  Australians  must  have  some  sort  of  "wolf,"  and  this  is  the 
nearest  approach  that  the  marsupials  can  afford.  It  is  a  predaceous 
animal,  almost  as  large  as  a  small  wolf,  with  a  dog-like  head  and  a 
series  of  tiger-like  bands  across  the  back  and  tail. 

Family  5.  Peramelidce  (Bandicoots). — There  are  three  genera  in 
this  family.  Perameles  is  a  genus  of  twelve  species  of  medium  sized 
forms,  with  the  pouch  opening  backwards.  Peragale  (Fig.  184,  F) 
is  a  genus  of  two  species  of  "rabbit  bandicoots,"  which  have  the 
habit  of  burrowing  in  the  soil  for  grubs  and  other  soil  insects.  Chcero- 
pus  castanotis  is  the  "pig-footed  bandicoot,"  also  a  burrowing  form, 
with  only  two  toes  on  the  fore  feet. 

Family  6.  Notoryctidce  (Marsupial  or  Pouched  Moles) . — This  fam- 
ily is  represented  by  a  single  species,  Notary ctes  typhlops  (Fig.  184,  E), 
a  South  Australian  mole-like  animal,  with  silky  reddish-gold  fur, 
which  harmonizes  with  the  color  of  the  arid  soil  in  which  it  burrows. 
It  has  a  complete  set  of  mole-like  adaptations  and  leads  a  thoroughly 
mole-like  life.  The  eyes  are  rudimentary;  there  are  no  external  ear 
lobes;  the  fore  feet  are  armed  with  extremely  heavy  burrowing  claws, 
the  third  and  fourth  being  much  more  conspicuous  than  the  rest; 
the  tail  is  very  short  and-  stumpy. 


MAMMALIA  357 

SUB-ORDER  2.  DIPROTODONTIA 

The  members  of  this  division  are  mainly  herbivorous.  Their 
dentition  is  not  unlike  that  of  the  rodents,  the  incisors  being  of  the 
gnawing  type,  usually  two  pairs  above  and  one  pair  below.  The 
canines  are  either  small  or  absent;  the  molars  have  either  tubercles 
or  transverse  ridges.  This  group  contains  the  largest  and  most  highly 
specialized  of  the  marsupials. 

Family  7.  Epanorthidce. — This  family  consists  of  various  extinct 
forms  and  the  single  living  genus  Ccenolestes  (marsupial  shrews),  the 
only  American  diprotodonts.  It  is  native  to  Andean  foot-hills  of 
South  America.  The  affinities  of  this  genus  are  still  somewhat  in 
doubt,  but  Osgood,  in  an  unpublished  monograph  on  the  genus,  claims 
that  it  is  in  a  sense  intermediate  between  the  polyprotodonts  and  the 
diprotodonts.  It  has  a  primitive  diprotodont  dentition,  but  a  foot 
structure  more  like  that  of  the  polyprotodonts.  Its  resemblances  to 
Perameles  are  rather  striking,  but  these  may  be  homoplastic  in 
character.  Osgood  considers  that  the  ancestor  of  Ccenolestes  was  a 
North  American  form,  which  also  may  have  given  rise  to  the  early 
diprotodont  stock  that  migrated  to  Australasian  territory.  In  gen- 
eral appearance  Ccenolestes  is  one  of  the  most  generalized  of  mar- 
supials, reminding  one  more  of  the  shrews  than  anything  else.  Many 
of  its  anatomical  features  are  also  very  generalized,  a  fact  that  is  in 
harmony  with  its  close  resemblance  to  a  long  extinct  group,  that 
lived  in  Miocene  times.  The  name  Ccenolestes  means  "a  modern 
representative  of  an  ancient  group." 

Family  8.  Phalangeridce  (Phalangers). — This  is  one  of  the  largest 
marsupial  families  and  consists  mostly  of  arboreal  forms.  They  are 
characterized  by  having  five  fingers  and  toes,  with  the  second  and 
third  phalanges  bound  together  by  an  integumentary  bond ;  the  hallux 
is  usually  opposable.  The  pouch  is  well  developed ;  the  tail  is  usually 
long.  The  following  are  some  of  the  more  important  genera:  Tarsipes, 
the  long-snouted  phalanger;  Acrobates,  the  pigmy  flying  phalanger; 
Distoechurus,  the  pen-tailed  phalanger;  Dromicia,  the  dormouse 
phalanger;  Petaurus,  the  true  flying  phalangers;  Tricosurus,  the  true 
phalangers;  Phascolarctus,  the  koala  or  marsupial  bear. 

The  true  phalangers  (Fig.  185,  F)  are  fairly  large  forms,  more  or 
less  fox-like  in  form  and  sometimes  known  as  "  brush-tailed  opos- 
sums." The  flying  phalangers  are  much  like  our  flying  squirrels  in 


358 


VERTEBRATE  ZOOLOGY 


FIG.  185.— Group  of  Marsupials  (Diprotodonts).  A,  Red  Kangaroo,  Macropus 
rufvs  (after  Lydekker);  B,  Rock  Wallabi,  Pelro$ale  xanthopus  (after  Vogt  and 
Specht;  C,  Young  Kangaroo  attached  to  nipple  in  pouch  of  mother;  pouch  laid 
back  to  show  interior  (after  Brehm);  D,  lateral  view  of  same  removed  from 
pouch  (after  Parker  and  Haswell) ;  E,  Koala,  Phascolarctos  cinereus,  carrying  young 
on  back  (after  Brehm) ;  F,  Phakmger  maculatus;  G,'  Wombat,  Phascolomys 
ur sinus  (after  Lydekker). 


MAMMALIA  359 

structure  and  habits;  they  are  not  genuine  flyers  but  merely  soarers 
that  parachute  from  tree  to  tree  by  means  of  folds  of  skin  stretched 
between  the  fore  and  hind  limbs.  The  koala  is  a  curious  slow-moving, 
nocturnal  animal,  that  feeds  almost  exclusively  on  the  leaves  of  the 
gum  tree.  It  has  been  called  "marsupial  bear,"  but  is  really  more 
like  a  large  "Teddy  Bear"  than  anything  else,  as  the  illustration 
(Fig.  185,  E)  plainly  attests. 

Family  9.  Macropodidce  (Kangaroos,  Wallabies,  etc.). — The  kan- 
garoos are  mostly  terrestrial  forms,  but  some  of  them  appear  to  be 
secondarily  arboreal.  The  hind  legs  are  very  large  and  powerful  and 
usually  the  fourth  and  fifth  toes  are  much  enlarged  into  a  sort  of  hoof. 
The  tail  is  always  long  and  heavy  at  the  base.  Macropus  rufus  (Fig. 
185,  A)  is  the  largest  of  the  marsupials,  attaining  a  length  of  five  and 
a  half  feet,  exclusive  of  the  tail.  They  are  very  fleet  of  foot,  progress- 
ing by  great  leaps  of  the  long  hind  legs  covering  twenty  feet  at  a 
jump.  The  fore  legs  are  of  no  use  in  running  and  appear  to  be  merely 
for  grasping  food  and  for  handling  the  young.  The  genus  Petrogale 
(Fig.  185,  B)  includes  kangaroos  that  live  among  the  rocks,  using 
the  long  tail  as  a  balancing  pole  as  they  leap  from  rock  to  rock. 
Dendrolagus  (the  tree  kangaroo)  is  very  different  in  its  habits  from 
any  of  the  other  members  of  the  family.  The  foot  structure  indicates 
that  the  arboreal  habit  has  been  superimposed  upon  an  ancestral 
cursorial  habit,  for  there  is  the  same  great  enlargement  of  the  fourth 
and  fifth  toes  as  in  the  other  kangaroos. 

Family  10.  Phascolomyidce  (Wombats). — This  family  consists  of 
but  one  genus,  Phascolomys.  It  is  in  general  appearance  something 
like  a  small  bear  (Fig.  185,  G)  or  a  heavily  built  marmot.  It  lives 
entirely  on  the  ground  and  moves  about  with  a  sort  of  shuffling 
plantigrade  gait  much  after  the  manner  of  a  bear.  It  is  shy  and 
gentle,  though  it  can  put  up  a  vigorous  defense  with  teeth  and  claws 
if  forced  to  do  so.  In  habits  it  is  nocturnal,  spending  the  daytime 
in  burrows  or  holes  among  the  rocks. 


CHAPTER  X 

MAMMALIA— Continued 

DIVISION  II.  MONODELPHIA  (PLACENTAL  MAMMALS) 

Definition. — This  is  the  great  group  of  present-day  mammals? 
including  about  95  per  cent,  of  all  living  mammalian  species.  They 
are  characterized  by  the  following  features:  no  marsupium;  no  epi- 
pubic  bones;  the  young  always  nourished  for  a  considerable  time  in 
the  uterus  by  means  of  a  placenta;  no  cloaca;  always  a  good-sized 
corpus  callosum. 

The  most  primitive  placental  mammals  are  now  believed  to  be 
more  nearly  representative  of  the  ancestral  mammalian  prototype 
than  are  the  monotremes  or  marsupials.  Certain  members  of  the 
order  Insectivora  have  been  selected  as  the  most  generalized  of  living 
mammals.  Osborn  selects  as  his  mammalian  prototype  the  tree  shrew 
Tupaia  (Fig.  186,  B),  while  Lull  selects  as  his,  Gymnura  (Fig.  186,  A), 
a  large  rat-like  animal  related  to  the  hedgehogs.  The  most  special- 
ized mammals  are  undoubtedly  the  whales,  if  structural  modification 
be  taken  as  the  criterion;  but  Man  outranks  all  other  mammals  in 
brain  and  nervous  specialization,  and  therefore  in  intelligence. 

SECTION  A.    UNGUICULATA  (CLAWED  MAMMALS) 

ORDER  I.     INSECTIVORA    (HEDGEHOGS,   MOLES  AND   SHREWS) 

These  are  primitive,  rather  small,  furry  animals,  that  feed  almost 
exclusively  on  insects.  They  are  for  the  most  part  nocturnal  and 
terrestrial  in  habit,  as  the  first  mammals  are  believed  to  have  been. 
Some  of  them  have  been  specialized  slightly  for  an  arboreal  habit; 
others  have  been  rather  profoundly  modified  for  a  fossorial  life.  In 
bodily  proportions  they  are  as  a  rule  quite  generalized,  fitting  well 
the  role  usually  assigned  to  them  of  persistently  primitive  mammals. 

The  members  of  the  Shrew  family  (Fig.  186,  A  and  B)  are  rather 
rat-like  in  form  and  more  or  less  plantigrade  in  attitude.  There  is 
nothing  especially  striking  or  noteworthy  about  these  animals  ex- 
cept their  lack  of  specialized  characters.  It  has  already  been  pointed 

360 


MAMMALIA 


361 


FIG.  186. — Group  of  Insectivora.  A,  Gymnura  rafflesii,  believed  by  Lull  to 
be  the  most  primitive  insectivore  (after  Horsfield  and  Vigors);  B,  Tupaia,  the 
Tree  Shrew,  considered  by  Osborn  as  near  the  prototype  form  of  all  higher  pla- 
cental  mammals  (after  Osborn);  C,  Golden  Mole,  Chrysochloris  trevelyani  (after 
Giinther).  (All  redrawn.) 


362 


VERTEBRATE  ZOOLOGY 


out  that  various  authorities  on  mammalian  morphology  have  selected 
the  shrews  as  the  most  generalized  of  living  mammals. 

The  Erinaceidoe — Erinaceus,  Hylomys,  and  Gymnura  (Fig.  186,  A) — 
are  a  little  more  specialized  than  are  the  shrews,  though  Lull  con- 
siders the  latter  the  most  primitive  living  placental  mammal.  The 
true  hedgehog  is  characterized  by  its  armor  of  quills,  which  are  much 
like  those  of  the  porcupine  in  structure. 


FIG.  187. — Galeopithecus. — (From  Parker  and  Haswell,  after  Vogt  and  Specht.) 

The  True  Moles  (Fig.  186,  C)  are  profoundly  specialized  for  a 
sub-terrestrial  burrowing  habit  and  resemble  in  their  adaptations 
the  marsupial  mole.  They  have  rudimentary  eyes,  no  ear-lobes, 
short  tail,  and  heavy  digging  claws.  The  golden  mole  (Chrysochloris) 
of  South  Africa  is  a  beautiful  creature  with  iridescent  golden  fur. 
Moles  feed  chiefly  on  earthworms  and  dig  long  tunnels  just 
beneath  the  turf,  and  on  this  account  are  the  bane  of  lawn- 
keepers  and  gardeners.  No  less  than  nine  families  of  Insectivora 


MAMMALIA  363 

have  been  distinguished,  but  lack  of  space  forbids  a  detailed  de- 
scription of  them. 

ORDER  2.  DERMOPTERA. — This  is  an  order  containing  but  a  single 
species,  Galeopithecus  volans  (Fig.  187),  the  so-called  "flying  lemur." 
It  is  a  bat-like  creature,  nearly  as  large  as  a  cat,  with  membranes 
stretched  between  the  fore  and  hind  legs,  also  between  the  head  and 
the  hand  and  between  the  tail  and  the  hind  feet.  In  certain  respects 
it  seems  to  be  intermediate  between  the  insectivores  and  the  bats. 

ORDER  3.  CHIROPTERA  (BATS). — Bats  may  be  defined  as  true  flying 
mammals  in  which  the  fingers  of  the  fore  limb  are  greatly  elongated 
to  support,  like  the  ribs  of  a  fan,  a  membraneous  airplane.  They  do 
not  merely  soar  or  parachute  like  the  flying  lemur  or, the  flying  squir- 
rels, but  actually  propel  themselves  with  rapid  wing  strokes  as  effec- 
tively as  do  many  of  the  birds.  Extra  planing  surface  is  acquired 
by  a  stretch  of  membrane  running  from  the  hind  limbs  to  the  tail. 
The  knees  of  bats  are  turned  backwards,  a  position  that  would  require 
dislocation  of  the  hip  in  any  other  mammal.  Many  of  the  bats  have 
large  delicate  ears  and  extremely  complicated  folds  of  sensitive  mem- 
brane surrounding  the  nostrils  (Fig.  188,  B,  C,  D);  these  are  believed 
to  be  organs  of  a  sixth  sense  (kinsesthetic  sense)  that  gives  warning 
of  the  nearness  of  solid  objects  in  the  dark.  It  is  said  that  bats  liv- 
ing in  caves  that  have  absolutely  no  light,  fly  about  in  swarms  at  a 
high  speed  and  never  collide  with  one  another  nor  with  the  walls 
or  roof  of  the  cave.  Bats  are  divided  into  two  sub-orders:  Micro- 
chiroptera  and  Megachiroptera. 

Sub-Order  1 .  Megachiroptera  (Fruit-eating  Bats) . — These  are  rather 
large  animals  and  are  sometimes  called  "flying  foxes."  They  occur 
in  India,  Australasia,  Ceylon,  Africa  and  Madagascar.  The  best 
known  is  Pteropus,  a  large  bat  with  a  wing-spread  of  over  five  feat, 
though  the  body  is  only  about  a  foot  in  length.'  Their  main  food  con- 
sists of  figs  and  guava.  They  are  distinctly  social  in  habit  and  move 
about  in  droves  of  considerable  size.  Another  well-known  species 
is  the  collared  fox-bat  (Xantharpyia  collaris)  which  is  shown  in  its 
customary  resting  position  with  its  young  clinging  to  its  abdomen 
(Fig.  188,  A). 

Sub-Order  2.  Microchiroptera  (Insectivorous  Bats). — These  are 
small  bats  (Fig.  188,  B)  with  practically  cosmopolitan  range  on  ac- 
count of  their  great  powers  of  flight.  At  least  five  hundred  species 
are  known.  They  are  decidedly  nocturnal  in  habit,  taking  up  the 


364  VERTEBRATE  ZOOLOGY 

role  of  birds  while  the  latter  are  asleep.  " Blind  as  a  bat"  is  a  fa- 
miliar aphorism  that  has  its  basis  in  the  fact  that  the  bats'  eyes  are 
so  sensitive  to  lights  of  low  intensity  that  they  are  blinded  by  the 
broad  daylight.  At  night  they  skim  rapidly  and  dexterously  through 
the  air  catching  insects  on  the  wing  with  remarkable  expertness.  In 
the  daytime  they  spend  their  time  sleeping  in  caves  or  other  dark 
sheltered  places,  hanging  up-side-down  by  means  of  the  claws  of 
their  hind  legs.  They  are  decidedly  gregarious,  living  in  colonies  of 
thousands  within  the  narrow  confines  of  certain  small  caves.  A  com- 
mon American  species  is  the  Brown  Bat  (Eptesicus  fuscus) ;  another 
common  species  of  the  eastern  parts  of  North  America  is  the  Little 
Brown  Bat  (Myotis  lucifugus),  which  is  less  than  three  and  a  half 
inches  in  length.  The  Vampire  (Desmodus  rotundus)  is  a  bat  of  rather 
large  size,  native  to  South  America.  True  to  its  reputation  it  lives 
the  life  of  a  blood-sucker,  attacking  horses  and  cattle  and  occasionally 
men.  Its  mode  of  attack  is  to  fasten  its  razor-edged  front  teeth 
(Fig.  188,  E)  in  the  throat  and  to  sever  a  vein  or  an  artery,  after  which 
it  proceeds  to  gorge  itself  with  blood.  One  curious  family  of  bats, 
the  Molossidce,  are  of  interest  because  they  have  become  secondarily 
terrestrial,  appearing  to  be  more  at  home  on  their  feet  than  one  would 
expect  of  a  bat;  for  they  run  about  almost  like  mice.  This  is  quite 
in  contrast  to  the  usual  situation  among  bats,  which  move  about  on 
the  land  with  extreme  awkwardness.  When  the  typical  bat  crawls 
it  hooks  the  thumb-nail  in  front  and  pushes  with  its  feet  behind,  a 
pitiably  helpless  mode  of  locomotion. 

ORDER  4.  CARNIVORA  (FLESH-EATING  MAMMALS). — This  is  an  im- 
mense order,  characterized  by  large  average  size,  predatory  habits,  and 
dominant  position  in  the  economy  of  nature.  The  largest  carnivores, 
lions,  tigers,  and  bears  r.ank  as  kings  among  beasts.  The  cheek  teeth 
are  generally  provided  with  sharp  cutting  edges,  and  the  canines  are 
characteristically  large  and  curved.  The  brain  is  relatively  large  and 
complex;  a  fact  that  accords  well  with  their  high  grade  of  intelligence. 
The  clavicle  is  vestigial  or  absent,  giving  them  a  narrow-chested  ap- 
pearance. Digits  are  never  less  than  four  and  are  armed  with  curved 
claws.  It  is  difficult  to  enter  into  a  more  detailed  account  of  the 
order  as  a  whole,  because  the  two  sub-orders,  Fissipedia  and  Pinni- 
pedia,  are  unlike  in  so  many  particulars. 

Sub-Order  1.  Fissipedia  (Terrestrial  Carnivores). — The  dentition 
(Fig.  169)  is  probably  the  best  diagnostic  character  of  this  group; 


MAMMALIA 


365 


FIG.  188. — Chiroptera  (Bats).  A,  Collared  Fox-Bat,  Xantharpyia  collaris,  and 
young.  (After  Sclater.)  B,  Synotus  barbastellus.  (After  Vogt  and  Specht.) 
C,  Face  of  Trmnops  persiciks,  showing  nasal  folds.  (After  Dobson.)  D,  Face 
of  Centurio  senex.  (After  Dobson.)  E,  Dentition  of  Vampire,  Desmodus  rufus, 
to  show  sharpness  of  teeth.  (After  Flower  and  Lydekker,.) 


366  VERTEBRATE  ZOOLOGY 

they  have  six  incisors  of  small  size  in  each  jaw,  canines  are  large  and 
strong,  the  last  premolar  and  the  first  molar  are  "carnassial "  or  cutting 
teeth,  and  the  last  two  molars  are  crushing  teeth.  The  fissiped  car- 
nivores have  a  world-wide  distribution,  being  native  to  all  of  the 
large  continental  bodies  except  Australia.  The  principal  family 
groups  are:  the  cats,  the  civets,  the  hyaenas,  the  dogs,  the  raccoons, 
the  weasels,  and  the  bears. 

Family  1.  Felidce  (Cats). — This  is  much  the  largest  and  most 
dominant  of  the  carnivore  families.  The  carnassial  teeth  are  highly 
perfected  shearing  organs,  canines  especially  long  and  curved,  and 
molars  are  greatly  reduced.  The  claws  are  retractile,  an  arrangement 
that  gives  the  cats  a  quiet  tread  when  stalking  their  prey.  The  typical 
genus  Felis  includes  such  cats  as  the  lions,  tigers,  leopards,  lynxes, 
jaguars,  ocelots,  pumas,  and  many  smaller  types.  The  domestic  cat 
is  believed  to  be  a  descendant  of  the  eastern  wild  species,  Felis  caffra, 
first  domesticated  by  the  Egyptians  and  considered  by  them  a  sacred 
animal.  The  Canada  lynx  (Fig.  189,  A)  is  a  short-tailed,  somewhat 
aberrant  type  of  cat. 

Family  2.  Viverridce  (Civets). — The  civets  (Fig.  189,  B)  and  their 
kin,  which  comprise  this  family  are  rather  small,  more  or  less  cat-like 
carnivores  that  are  native  to  Ethiopian  and  Oriental  regions.  The 
claws  are  incompletely  retractile  and  they  have  more  teeth  than  the 
true  cats.  The  civets  proper  are  decidedly  feline  in  appearance  and 
are  usually  marked  with  black  and  white  spots  or  stripes.  The  fossa 
is  a  very  cat-like  carnivore;  it  is  the  largest  carnivore  native  to  Mada- 
gascar. The  mongoose  is  a  small,  extremely  active  animal  of  oriental 
countries;  it  is  noted  for  its  ability  to  kill  snakes,  especially  the  deadly 
cobra. 

Family  3.  Hycenidce  (Hyoenas) . — These  animals  (Fig.  189,  C)  are 
in  appearance  and  habits  intermediate  between  the  cats  and  the  dogs. 
They  are  either  spotted  or  striped.  The  voice  is  said  to  be  almost 
human  in  sound  and  stories  are  told  of  human  beings  lured  to  their 
death  by  following  their  cries. 

Family  4-  Canidce  (Dogs) . — The  dog  family  includes  the  wolves 
(Fig.  189,  D),  foxes,  coyotes,  and  the  dingo  of  Australia,  which  is 
believed  to  be  an  imported  species.  The  domestic  dogs  are  believed 
to  have  been  derived  from  several  wild  stocks,  some  of  which  may 
have  become  extinct.  In  many  ways  the  dogs  are  the  most  primitive 
of  the  carnivores:  the  dentition  is  quite  generalized,  the  claws  are 


FIG.  189. — Group  of  Fissiped  Carnivora.  A,  Canada  Lynx,  Felis  canadensis 
(after  Fuertes).  B,  Civet  Cat,  Viverra  civeita  (after  Beddard).  C,  Spotted 
Hyaena,  Crocula  maculala  (after  Beddard).  D,  Gray  or  Timber  Wolf,  Canis 
nubilus  (after  Fuertes).  E,  Raccoon,  Procyon  lotor  (after  Fuertes).  F,  Badger, 
Taxidea  taxus  (after  Fuertes).  G,  Otter,  Lulra  canadensis  (after  Fuertes).  H, 
Largest  of  the  bears,  Alaska  Brown  Bear,  Ursus  gyas  (after  Fuertes.)  (All  figures 
redrawn,  those  after  Fuertes  in  National  Geographic  Magazine,  simplified  and 
more  or  less  modified.) 

367 


368  VERTEBRATE  ZOOLOGY 

less  specialized  than  in  other  groups  and  in  several  other  ways  they 
appear  to  resemble  the  ancestral  carnivores.  They  have  been  as- 
sociated with  Man  from  a  very  early  period,  and  are  as  cosmopolitan 
in  their  distribution  as  Man  is,  because  wherever  Man  goes  he  takes 
his  dogs. 

Family  5.  Procyonidce  (Raccoons). — This  is  an  American  family  oc 
carnivores  that  in  some  ways  is  intermediate  between  the  dogs  and 
the  bears.  They  have  plantigrade  feet  and  grinding  teeth  like  the 
bears,  but  in  other  respects  are  more  like  the  dogs.  The  common 
raccoon  (Procyon)  is  a  familiar  type  (Fig.  185,  E)  around  streams  and 
lakes,  where  it  catches  crayfish,  clams,  and  sometimes  fish,  without, 
however,  going  very  far  into  the  water. 

Family  6.  Mustelidce. — This  is  a  large  family  of  bloodthirsty, 
predaceous  creatures,  including:  weasels,  pole-cats,  badgers  (Fig. 
189,  F),  martens,  wolverines,  sables,  minks,  ermines,  ferrets,  stoats, 
skunks,  otters  (Fig.  189,  G),  and  other  less  known  types.  For  the 
most  part  they  give  off  a  nauseous  musky  odor,  which  is  most  marked 
in  the  skunks.  They  are  among  the  most  important  of  our  fur-bearing 
animals.  Representatives  of  the  family  are  native  to  all  the  con- 
tinental bodies  except  Australia  and  Madagascar. 

Family  7.  Ursidce  (Bears).— The  bears  (Fig.  189,  H)  are  the  largest 
of  modern  carnivores  and  are  characterized  most  sharply  by  their 
plantigrade  walk  and  the  short  tail.  Most  bears  belong  to  the  genus 
Ursus,  but  several  other  genera  are  distinguished,  such  as  Melurus, 
the  sloth  bear  of  India,  and  sEluropus,  a  rare  species  native  to  Thibet. 
The  bears  are  native  to  the  Northern  Hemisphere,  few  of  them 
having  crossed  the  equator. 

Sub-Order  2.  Pinnipedia  (Seals  and  Walruses) . — The  animals  of 
this  sub-order  are  marine  forms,  in  which  there  has  been  a  secondary 
adaptation  of  the  whole  body  for  aquatic  life.  They  are,  however, 
much  less  radically  modified  than  the  Sirenia  or  the  Cetacea.  The 
Pinnipedia  are  characterized  as  follows:  the  greater  part  of  the  limbs 
are  inclosed  within  the  body  skin;  the  claws  are  reduced  and  the 
digits  are  increased  in  number;  the  milk  dentition  is  feeble  and  is 
shed  early;  the  cranial  cavity  is  large  as  compared  with  the  face. 

Family  1.  Otariidce  (Sea-lions  and  Fur-seals). — These  animals  are 
gregarious  and  polygamous.  The  males  (Fig.  190,  B)  are  several 
times  as  large  as  the  females  (Fig.  190,  C).  As  a  rule  they  breed  on 
rocky  northern  islands;  arid  great  numbers  have  in  the  past  been 


MAMMALIA 


369 


slaughtered  at  this  season.  The  governments  of  several  nations  have 
protected  seals  in  their  rookeries,  and  they  are  now  multiplying  satis- 
factorily. 

Family  2.  Trichechidce  (Walruses). — These  are  large,  heavy-bodied 
forms  (Fig.  190,  A)  with  tusk-like  canines  in  the  upper  jaws  and  a 


FIG.  190. — Pinniped  Carnivora.  A,  Pacific  Walrus,  Odobenus  obesus;  B,  Male, 
and  C,  female,  of  Steller  Sea-lion,  Eumetopias  jubata;  D,  Greenland  Seal,  Phoca 
groenlandica.  (All  redrawn  after  Fuertes.) 

mustache  of  heavy  bristles  on  the  upper  lip.  They  are  Arctic  in 
habitat.  On  the  whole  they  are  more  extensively  modified  for  aquatic 
life  than  are  the  sea-lions. 


370  VERTEBRATE  ZOOLOGY 

Family  3.  Phocidce  (The  True  Seals). — These  animals  have  no  ex- 
ternal ears;  the  nostrils  are  dorsal  in  position;  the  hind  limbs  are  in- 
timately bound  up  with  the  short  tail  to  make  a  sort  of  caudal  fin, 
which  is  used  as  a  very  effective  swimming  organ.  The  fore  limbs 
are  rather  small  and  fin-like,  and  the  whole  body  is  decidedly  spindle- 
shaped.  The  seals  are  much  more  highly  specialized  for  marine  life 
than  are  either  the  sea-lions  or  the  walruses.  One  of  the  commonest 
of  the  seals  is  Phoca  groenlandica  (Fig.  190,  D),  a  small  spotted  ani- 
mal about  four  or  five  feet  long. 

Extinct  Carnivores 

Representatives  of  the  MustelidaD  have  been  found  as  far  back  as 
Eocene  times;  some  Canidae  lived  during  Pliocene  times.  A  whole 


FIG.  191. — Extinct  carnivore,  Smilodon.    (From  Lull,  after  Knight  and  Osborn.) 

family  of  cat-like  creatures,  the  Machcerodontia,  lived  from  Eocene 
to  Pleistocene  times  and  are  now  extinct.  The  classic  "  saber-tooth  " 
(Smilodon)  is  a  characteristic  example  of  this  rather  remarkable  ex- 
tinct family  (Fig.  191),  which  was  characterized  mainly  by  the  ex- 
treme modification  of  the  teeth  and  skull  in  adaptation  to  the  peculiar 
method  of  attacking  with  the  saber-like  upper  canines.  These  huge 
teeth  were  thin  and  knife-like,  with  sharp  edges.  The  method  of 
using  these  teeth  was  evidently  quite  different  from  that  employed 
by  tigers;  the  prey  was  struck  a  downward,  slashing  blow,  and  was 
probably  stabbed  as  though  by  a  dagger.  The  upper  jaw  was  es- 
pecially modified  to  support  these  huge  canine  teeth,  and  the  skull 
was  radically  altered  to  furnish  attachment  for  the  huge  neck  muscles 


MAMMALIA  371 

that  were  used  in  driving  in  the  daggers.     The  lower  canines  are 
much  reduced  in  size. 

ORDER  5.  RODENTIA  (GNAWING  MAMMALS). — The  rodents  are  for 
the  most  part  rather  small  mammals,  though  a  few  of  them  have 
reached  a  considerable  size.  It  has  been  claimed  by  some  authorities 
that  there  are  more  species  of  rodents  living  to-day  than  of  all  other 
mammals  combined.  Unquestionably  they  are  the  most  typical  mam- 
malian group  to-day,  as  well  as  the  most  successful.  Because  they  are 
so  extremely  prolific,  because  they  are  omnivorous,  and  because  many 
of  them  lead  a  nocturnal  burrowing  life,  they  seem  likely  to  be  the 
main  mammalian  rivals  of  Man  in  the  next  geological  period.  The 
rodents  are  characterized  by  absence  of  canine  teeth ;  and  the  incisors 
are  long  and  strong,  with  persistently  growing  pulp  and  enamel  con- 
fined chiefly  to  the  anterior  edge.  This  arrangement  of  the  enamel 
makes  the  teeth  wear  down  to  a  chisel  edge,  which  is  self-sharpening 
with  use.  The  brain  is  smooth,  with  few  furrows,  and  the  intelligence 
is  usually  low.  The  testes  are  usually  abdominal  in  position;  the 
placenta  is  discoidal  and  deciduate.  Two  sub-orders  are  distin- 
guished: Duplicidentata  (Hares  and  Pikas)  and  Simplicidentata 
(Rodents  Proper). 

Sub-Order  1.  Duplicidentata  (Hares  and  (Pikas). — These  animals 
are  characterized  by  two  pairs  of  incisor  teeth  in  the  upper  jaw,  the 
inner  being  small  and  lying  behind  the  outer.  The  tail  is  short.  The 
group  is  regarded  by  some  as  a  distinct  order. 

Family  1.  Leporidce  (the  Hares)  are  distinguished  by  long  ears, 
long  hind  legs,  and  short  though  obvious  tail. 

Family  2.  Lagomyidce  (Pikas)  are  distinguished  by  short  ears,  short 
hind  legs,  and  no  external  evidences  of  a  tail. 

Sub-Order  2.  Simplicidentata  (True  Rodents). — The  members  of 
this  sub-order  are  divided  into  three  sections:  represented  by  squirrel- 
like,*  rat-like,  and  porcupine-like  rodents. 

Section  1.  Sciuromorpha  (Squirrel-like  Rodents). — This  large  sec- 
tion includes  the  squirrels  proper,  the  flying  squirrels,  the  ground 
squirrels  and  chipmunks,  the  gophers,  the  prairie  dogs,  the  marmots, 
the  beavers,  and  others.  The  flying  squirrels  (Fig.  192,  A)  are 
parachuting  animals,  with  a  membrane  stretched  between  the  fore 
and  hind  limbs.  The  prairie-dogs  are  burrowing  rodents  of  the  west- 
ern plains,  that  live  in  large  colonies  and  share  their  burrows  with 
ground  owls  and  rattlesnakes,  as  well  as  other  messmates.  The  habits 


372 


VERTEBRATE  ZOOLOGY 


of  the  beaver  (Fig.  192,  D)  are  too  well  known  to  require  description 
here.  They  are  nearing  extinction  on  account  of  their  highly  desirable 
fur.  No  other  rodent  is  so  highly  modified  for  aquatic  life  as  is  the 
beaver. 


FIG.  192. — Group  of  Rodentia.  A,  Flying  Squirrel,  Sriuroplerus  volucella  (after 
Lydekker);  B,  Long-tailed  Marmot,  Aiclomys  caudalus  (after  Beddard);  C, 
Egyptian  Jerboa,  Dipus  jaculus  (after  Lydekker);  D,  Beaver,  Castor  fiber  (after 
Lydekker);  E,  Agouti,  Dasyprocta  aguti  (after  Beddard);  F,  European  Por- 
cupine, Hystrix  cristata  (after  Beddard.) 


MAMMALIA  373 

Section  2.  Myomorpha  (Rat-like  Rodents) . — This  is  the  largest  mod- 
ern group  of  mammals  in  point  of  numbers  of  species  and  of  individ- 
uals. At  least  a  hundred  genera  and  nearly  five  hundred  species 
have  been  distinguished.  The  group  includes:  dormice,  field-mice, 
rats  and  mice  proper,  mole  rats,  jumping  mice  (Fig.  192,  C),  and  the 
so-called  African  flying  squirrels.  They  exhibit  a  very  wide  range  of 
adaptive  specializations,  being  terrestrial,  sub-terrestrial,  arboreal, 
cursorial  and  jumping,  aquatic,  and  volant.  They  do  serious  damage 
to  the  world's  food  supply  and  are  responsible  for  the  spread  of  some 
of  the  worst  plagues  that  Man  has  to  contend  with. 

Section  3.  Hystricomorpha  (Porcupine-like  Rodents). — This  is  a 
somewhat  heterogeneous  group  and  is  not  very  well  described  as 
"porcupine-like,"  since  many  types  appear  quite  unlike  porcupines. 
There  are  eight  families,  including:  " water-rats,"  cavies,  guinea-pigs, 
agoutis  (Fig.  192,  E),  chinchillas,  ground  porcupines,  and  tree  por- 
cupines. The  cavies  are  South  American  and  West  Indian  forms 
that  reach  a  length  of  four  or  five  feet.  They  are  terrestrial  in  habit, 
with  small  ears  and  short  tail.  The  chinchilla  is  a  small  squirrel-like 
animal  native  to  the  Andes;  the  fur  is  soft  and  gray  and  is  highly 
prized.  The  Canada  porcupine  is  a  heavy-bodied  terrestrial  and 
arboreal  form  that  gnaws  off  the  bark  of  trees,  eats  water-lily  leaves 
and  roots.  It  is  armed  with  short  quills  that  are  nearly  hidden  in  the 
long  fur.  Its  equipment  is  purely  for  passive  defense,  except  that  it 
lashes  the  tail  and  thus  drives  in  its  largest  quills,  when  attacked. 
Dogs  are  often  injured  when  they  are  unwise  enough  to  attack  the 
porcupine,  for  they  get  their  mouths  full  of  barbed  quills  that  are 
extremely  difficult  to  remove.  The  European  porcupine  (Fig.  192,  F) 
is  considerably  larger  than  its  American  relative,  having  a  body 
length  of  about  three  feet.  It  has  quills  nearly  a  foot  in  length,  those 
on  the  tail  being  hollow  so  as  to  produce  a  rattling  sound  when  the 
animal  is  disturbed.  A  great  crest  of  coarse  hair  surmounts  the  head 
and  hangs  down  like  a  mane.  In  spite  of  the  prevalent  reports  to 
that  effect,  the  porcupine  never  shoots  its  quills. 

ORDER  6.  EDENTATA  (SLOTHS,  ARMADILLOS,  AND  ANT-BEARS). — 
This  group  is  believed  to  be  a  surviving  remnant  of  an  archaic  group. 
They  have  become  highly  specialized  in  several  ways  and  exhibit 
many  evidences  of  racial  senescence.  The  name  of  the  order  implies 
a  total  lack  of  teeth  and  is  therefore  not  appropriate  for  either  the 
armadillos  or  the  sloths;  the  ant-bears  alone  are  quite  toothless. 


374  VERTEBRATE  ZOOLOGY 

The  dentition  of  the  toothed  edentates  is  peculiar  in  that  there  are 
no  incisors  and  the  teeth  in  the  definitive  condition  are  without 
enamel.  The  testes  are  abdominal;  the  clavicle  is  always  present; 
there  is  an  additional  pair  of  zygopophyses  on  the  posterior  dorsal 
and  lumbar  vertebrae.  The  edentates  are  strictly  American  in  dis- 
tribution and  have  been  limited  to  this  territory  from  the  first.  In 
adaptive  characters  the  three  main  types  differ  widely  from  one 
another. 

Sub-Order  1.  Pilosa  (Hairy  Edentates). — The  hairy  edentates  be- 
long to  two  quite  distinct  families:  The  Myrmecophagidse  (ant-bears), 
and  Bradipodidse  (sloths). 

Family  1.  Myrmecophagidoe  (Ant-bears). — These  are  among  the 
strangest  animals  now  living.  They  are  truly  edentate,  have  a  long 
slender  snout,  long  sticky  tongue,  heavy  front  claws,  and  long,  coarse 
hair,  characters  that  we  have  already  found  to  be  adaptive  features 
of  the  anteating  type  of  mammal,  no  matter  to  what  group  it  belongs. 
Myrmecophaga  tridactyla,  the  great  ant-bear,  is  a  large  animal  with  a 
total  length  from  end  of  snout  to  tip  of  tail  of  at  least  seven  feet.  It 
is  very  powerful  and  quite  formidable  when  attacked.  One  swipe  of 
the  great  hooked  claws  has  been  known  completely  to  eviscerate  a 
large  dog.  M,  jubata  (Fig.  193  A)  is  somewhat  smaller  but  quite 
similar.  The  Tamandua  is  a  smaller  ant-bear  with  arboreal  habits 
and  a  long  prehensile  tail.  Cyclopes  is  the  smallest  of  the  ant-bears. 

Family  2.  Bradipodidce  (Sloths)  .—The  sloths  (Fig.  193,  B),  in 
spite  of  their  marked  external  differences,  exhibit  many  fundamental 
resemblances  to  the  ant-bears.  They  are  highly  specialized  for  ar- 
boreal life.  Their  strong  hooked  claws  which  are  much  like  those 
of  the  ant-bears  are  used  as  hooks  for  suspending  them  from  branches. 
They  always  progress  up-side-down,  hanging  from  the  under  side  of  a 
branch.  In  accord  with  this  habitually  inverted  position  the  heavy 
hair  slopes  from  the  belly  toward  the  back;  similarly  the  hair  on  the 
limbs  slopes  from  the  feet  towards  the  body.  It  seems  likely  that 
this  peculiar  position  of  the  hair  serves  the  purpose  of  effectually 
shedding  the  rain.  An  interesting  fact  has  been  discovered  about  the 
hair:  it  is  green  in  color,  due  to  the  presence  in  the  hollows  of  the  in- 
dividual hairs  of  numerous  cells  of  a  green  alga.  This  greenish  color- 
ing doubtless  serves  as  a  protective  adaptation.  The  face  of  the 
sloth  is  extremely  flat,  in  very  marked  contrast  with  the  elongated 
face  of  the  ant-bears.  There  are  only  four  or  five  teeth  in  each  half 


MAMMALIA 


375 


jaw.  The  sloths  are  very  peculiar  in  that  they  have  an  excessive 
number  of  dorsal  vertebrae,  as  many  as  23  being  present  in  some 
species.  The  cervical  vertebrae  are  also  quite  an  exception  for  mam- 


FIG.  193. — Edentata,  Pholidota,  and  Tubulidentata.  A,  Great  Anteater, 
Myrmecophaga  jubata;  B,  Two-toed  Sloth,  Cholcepus  didactylus;  C,  Texas  Nine- 
banded  Armadillo,  Dasypus  novemcinctus  texanus;  D,  The  Aard  Vark,  Orycteropus 
capensis;  E,  Short-tailed  Pangolin,  Manis  temmincki.  (All  redrawn,  A,  D,  E, 
after  Lydekker;  B,  after  Beddard;  C,  after  Newman.) 


376  VERTEBRATE  ZOOLOGY 

mals,  in  that  they  depart  consistently  from  the  number  seven,  which 
is  so  characteristic  for  mammals,  having  six,  eight,  or  nine.  They 
are  largely  insectivorous  in  diet.  Bradypus,  the  three-toed  sloth,  and 
Chcelopus,  the  two- toed  sloth,  are  the  best  known  members  of  the 
family. 

Sub-Order  2.  Loricata  (Armored  Edentates;  Armadillos). — The  liv- 
ing armadillos  belong  to  the  family  Dasypodidce  and  are  much  more 
numerous  in  species  than  are  the  Pilosa.  At  least  seven  genera  and 
over  twenty  species  have  been  distinguished.  They  are  characterized 
by  having  a  well-developed  dermal  skeleton,  composed  of  numerous 
bony  plates,  in  which  hairs  are  imbedded,  and  which  are  covered  with 
horny  scales.  They  have  numerous  teeth,  which  in  the  adult  are 
without  enamel;  but  in  the  embryonic  stages  a  well-defined  enamel 
layer  has  been  discovered,  which  subsequently  wears  off.  Incisors 
are  not  found  in  the  adult,  but  embryonic  rudiments  of  these  teeth 
have  been  described.  The  armadillos  range  from  moderately  large 
animals  of  three  feet  or  more  in  length  to  small  forms  about  the  size 
of  a  rat.  Only  a  few  of  the  species  can  receive  mention  here.  The 
little  Chlamydophorus  has  a  solid  unjointed  armature  and  is  consid- 
ered primitive  in  this  respect.  Euphractus  sexcinctus  (the  Peludo) 
is  a  decidedly  hairy  type.  Tolypeutes  has  three  movable  bands  and 
rolls  up  into  a  ball.  Priodontes  is  the  giant  among  armadillos,  being 
three  feet  long  to  the  base  of  the  tail  and  having  thirteen  movable 
bands  in  the  armor. 

Dasypus  novemcinctus  (the  nine-banded  armadillo)  is  the  only 
North  American  armadillo  and  therefore  deserves  especial  attention. 
It  is  really  a  South  American  species  that  has  migrated  northward 
through  Central  America  and  now  inhabits  Mexico  and  Southern 
Texas.  It  is  a  medium  sized  animal  that  lives  in  burrows  in  the  day- 
time and  forages  for  insects  at  night.  Its  ears  are  long  and  close  to- 
gether and  remind  one  of  a  donkey's  ears.  It  is  a  source  of  satisfac- 
tion to  be  able  to  contribute  an  adequate  illustration  (Fig.  193,  C) 
of  this  interesting  species  to  take  the  place  of  the  atrocious  figure  of 
Flower  and  Lydekker,  which  was  evidently  drawn  from  a  badly  stuffed 
specimen.  Perhaps  this  armadillo  deserves  especial  mention  on  ac- 
count of  its  unique  embryological  features.  It  produces  regularly, 
with  rare  exceptions,  four  young  at  a  birth,  that  are  always  all  four 
of  the  same  sex.  A  study  of  the  early  developmental  history  of  the 
egg  has  revealed  the  fact  that  this  is  a  case  of  specific  polyembryonyt 


MAMMALIA  377 

in  which  the  four  individuals  are  produced  from  a  single  fertilized  egg, 
that  divides  at  a  very  early  period  into  four  embryos.  There  is  a 
single  chorion,  but  four  separate  amnia.  This  case  is  taken  as  evi- 
dence that  in  mammals  sex  is  determined  at  the  time  of  fertilization, 
since  the  four  division  products  of  a  single  egg  are  invariably  of  the 
same  sex. 

Extinct  Edentata. — The  best  known  extinct  edentates  are  the 
giant  ground  sloths,  of  which  Mylodon  is  a  type,  and  the  giant  arma- 
dillos, of  which  Glyptodon  is  the  classic  example.  Mylodon  was  as 
large  and  as  heavy  as  a  rhinoceros,  and  Glyptodon  was  sixteen  feet 
long. 

ORDER  7.  PHOLIDOTA  (SCALY  ANTEATERS). — This  is  a  small  order 
formerly  included  within  the  order  Edentata,  but  now  given  ordinal 
value  on  account  of  the  discovery  of  morphological  differences  more 
fundamental  than  the  resemblances  that  formerly  led  to 'their  classifi- 
cation as  edentates.  The  order  consists  of  the  pangolins,  which  are 
placed  in  the  family  Manidce  and  the  genus  Manis.  Manis  gigantea  is  a 
fairly  large  and  massive  animal,  about  six  feet  in  length,  tail  included. 
It  is  native  to  the  islands  of  the  Malayan  Archipelago.  The  most 
striking  feature  of  these  animals  is  the  scaly  covering,  or  what  ap- 
pears to  be  an  armor  composed  of  large  pointed,  overlapping  scales, 
which  are  really  groups  of  fused  hairs.  Scattered  hairs  occur  between 
these  "scales."  The  species  shown  in  the  illustration  is  Manis  tern- 
minckii  (Fig.  193,  E). 

The  pangolins  are  anteaters,  and  possess  all  of  the  characteristic 
adaptations  already  mentioned  for  several  other  anteaters:  the  long 
snout,  sticky  tongue,  integumentary  protection  from  ants,  and  heavy 
claws  for  digging  into  ant  galleries.  Their  method  of  capturing  ants 
is  highly  individual.  After  stirring  up  a  colony  of  ants  they  are  said 
to  erect  the  scales  so  as  to  allow  ants  to  crawl  under  the  scales.  The 
scales  are  then  clamped  down  so  as  to  hold  the  ants,  and  then  the 
animal  goes  in  for  a  swim.  When  submerged  in  the  water  the  scales 
are  lifted  and  the  ants  washed  out  so  that  they  float  about  on  the 
surface,  where  they  are  easily  picked  up  by  means  of  the  long  tongue. 

ORDER  8.  TUBULIDENTATA. — This  order  contains  only  the  curious 
aard-vark,  Orycteropus  (Fig.  193,  D)  of  South  Africa.  These  curious 
animals  were  formerly  classed  as  edentates,  but  are  now  known  to 
be  unique  in  a  number  of  characters  and  have  therefore  been  ac- 
corded ordinal  value.  They  are  anteaters  and  have  the  slender 


378  VERTEBRATE  ZOOLOGY 

snout,  long  tongue,  and  strong  claws  characteristic  of  this  habitus. 
The  skin  is  very  thick  and  is  covered  with  sparse  hair. 

SECTION  B.  PRIMATES  (MAMMALS  WITH  "NAILS") 

ORDER  9.  PRIMATES  (LEMURS,  MONKEYS,  APES  AND  MAN). — The 
traditional  position  allotted  to  the  primates  is  the  last  and  highest  or- 
der of  mammals,  but  it  has  come  to  be  realized  that  the  group  is  on 
the  whole  more  generalized  than  several  other  orders,  and  in  point  of 
specialization  belongs  to  a  division  just  above  that  rather  primitive 
section,  Unguiculata.  The  primates  may  be  defined  as  primarily 
arboreal  animals  with  prehensile  limbs;  with  thumb  and  great  toe 
shorter  than  the  other  digits  and  more  or  less  opposable  to  the  latter; 
with  plantigrade  walking  position  of  the  feet;  with  terminal,  flat- 
tened " nails"  instead  of  claws;  with  hair  covering  the  entire  body 
except  the  palms  and  soles  and  parts  of  the  face;  with  a  single  pair  of 
pectoral  mammae;  with  the  eyes  directed  anteriorly  instead  of  later- 
ally; the  eye  orbit  completely  surrounded  with  bone;  a  clavicle  always 
present;  the  stomach  simple;  and  the  brain  unusually  large  and  well 
convoluted. 

Probably  the  best  among  many  classifications  of  the  primates  is 
that  of  W.  K.  Gregory: 

Sub-Order  1.  Lemur oidea  (lemurs  or  " half-apes"). 
Sub-Order  2.  Anthropoidea. 

Series  1.  Platyrrhini  (New  World  Apes). 
Family  1.  Hapalidse  (Marmosets). 

"     2.  Cebidse   (capuchins,   howler  monkeys,   spider 

monkeys,  etc.). 
Series  2.  Catarrhini  (Old  World  Apes  and  Monkeys). 

Family  3.  Cercopithecidee  (monkeys,  baboons,  macaques, 

etc.). 

"      4.  Simiidae  (man-like  or  anthropoid  apes). 
"      5.  Hominidae  (men). 

Sub-Order  1.  Lemuroidea  (Lemurs). — The  lemurs  (Fig.  194,  A)  are 
much  the  most  ancient  and  the  most  generalized  of  the  primates,  and 
therefore  show  less  wide  departures  from  the  unguiculate  mammals 
than  do  the  anthropoids.  They  are  exclusively  arboreal,  mostly 
nocturnal,  and  extremely  timid  and  retiring.  In  appearance  they 
strike  one  as  intermediate  between  a  squirrel  and  a  monkey.  The 


MAMMALIA 


379 


brain  is  comparatively  unspecialized,  the  cerebral  hemispheres  being 
so  small  as  not  to  cover  the  hind-brain.    The  second  finger  retains 


'£. 


FIG.  194. — Group  of  Primates.  A,  Smith's  Dwarf  Lemur.  Miaocebus  smithii; 
B.  Spider  Monkey,  Aides  ater;  C,  Drill  or  Mandrill,  Papio  leucophceus;  D,  Gib- 
bon, Hylobates  lar;  E  and  F,  Chimpanzee,  Pan  pygmctus.  (Redrawn,  A  and  B, 
after  Beddard;  rest  after  Lydekker.) 

the  ancestral  claw,  but  the  rest  of  the  fingers  have  "nails."  The 
lemurs  have  their  headquarters  in  Madagascar,  but  are  also  found 
in  small  numbers  in  the  tropical  forests  of  Africa  and  in  Malaysia. 


380  VERTEBRATE  ZOOLOGY 

During  the  Eocene  period  they  lived  both  in  North  America  and  in 
Europe,  a  fact  indicative  of  the  antiquity  of  the  group.  Two  per- 
sistent relics  of  that  Eocene  lemuroid  fauna  are  the  living  genera 
Tarsius  and  Chiromys. 

Chiromys  madagascariensis,  the  "aye-aye,"  is  a  rather  squirrel-like 
animal  with  long  incisor  teeth;  a  bushy  tail;  the  thumb  only  has  a 
"nail,"  the  other  digits  being  provided  with  claws;  the  mammae  are 
abdominal,  a  primitive  position;  it  has  but  one  young  at  a  birth.  The 
"aye-aye"  has  a  plaintive  voice  resembling  the  name;  it  leads  a 
prowling,  furtive  life,  always  in  pairs.  A  nest  of  twigs  is  made  in  the 
tops  of  trees. 

Tarsius  spectrum,  a  native  of  the  Malay  Islands,  is  a  remarkably 
strange  little  creature,  with  enormous  eyes  that  give  it  the  appear- 
ance of  wearing  spectacles,  a  character  from  which  it  derives  its 
specific  name.  The  digits  are  armed  with  adhesive  pads  and  have 
small  flat  nails.  The  tail  is  long  and  tufted  at  the  end.  They  live 
in  pairs  in  holes  in  hollow  trees,  and  are  mainly  insectivorous  and 
decidedly  nocturnal.  The  mother  carries  the  young  about  by  taking 
hold  of  the  neck  skin  with  the  teeth,  after  the  manner  of  a  mother  cat. 
Tarsius  has  an  almost  smooth  cerebrum  and  a  low  order  of  intelligence. 

The  more  modernized  lemurs  may  be  exemplified  by  the  ruffed 
lemur,  the  mouse  lemur,  and  the  slow  loris.  Of  all  the  lemurs  the 
ruffed  lemur  (Lemur  varius)  is  probably  the  most  monkey-like.  It 
has  a  rather  long,  bushy  tail,  a  fox-like  face  and  the  full  primate 
dentition.  The  voice  is  loud;  they  are  diurnal  as  well  as  nocturnal 
in  habit.  The  mouse  lemur  (Chirogale  coquereli)  is  a  native  of  Mada- 
gascar; it  is  very  small  in  size,  with  soft,  fluffy  fur  and  of  generalized 
proportions.  The  slow  loris  (Nycticebus  tardigradus)  is  an  aberrant 
lemur,  native  of  East  Indian  and  Malayan  territories.  It  is  extremely 
deliberate  in  its  movements,  moving  about  among  the  trees  chatter- 
ing and  whistling  as  though  without  a  care  in  the  world.  Like  other 
lemurs  it  is  looked  upon  with  superstitious  dread  by  the  natives,  who 
regard  it  as  a  beast  of  ill  omen. 

Sub-Order  2.  Anthropoidea  (Monkeys,  Apes,  Man). — The  anthro- 
poids are  decidedly  more  highly  organized  than  are  the  lemurs.  They 
are  characterized  by  the  possession  of:  32  to  36  teeth;  completely 
closed  orbit;  pectoral  mammse;  prehensile  hands  and  feet  (except  in 
Man);  cerebral  hemispheres  richly  convoluted  and  covering  the 
cerebellum. 


MAMMALIA  381 

Series  1.  Platyrrhini  (New  World  Apes). — These  primates  are  dis- 
tinguished by  the  broad  nasal  septum;  the  thumb  is  not  opposable, 
but  usually  reduced  to  a  small  vestige;  the  tail  is  long  and  prehensile; 
there  are  no  cheek  pockets  or  pouches;  there  are  no  callosities  on  the 
ischium. 

Family  1.  Hapalidce. — These  are  the  marmosets,  animals  about 
the  size  of  squirrels,  quite  extensively  used  as  pets.  They  have  a 
very  generalized  diet,  eating  fruit,  eggs,  and  insects,  and  have  claws 
instead  of  nails  on  the  digits. 

Family  2.  Cebidce. — Most  of  the  common  South  American  mon- 
keys (Fig.  194,  B)  belong  to  this  family.  Several  species  of  them  are 
familiar  to  everyone  as  the  accessory  of  the  Italian  organ-grinder. 
They  are  all  rather  slender  and  have  exceptionally  long,  more  or  less 
prehensible  tails.  The  howler  monkeys  are  noted  for  their  prodigious 
voice,  which  is  produced  by  means  of  a  specially  modified  sounding 
apparatus.  The  commonest  of  the  Cebidse  are  the  capuchins,  com- 
panions of  the  hand-organ. 

Series  2.  Catarrhini  (Old  World  Apes  and  Man). — This  series  of 
primates  is  characterized  by:  narrow  nasal  septum,  with  nostrils 
directed  downward;  all  have  32  teeth,  as  in  man;  non-prehensile  or 
rudimentary  tail;  the  great  toe  fully  opposable,  except  in  man;  the 
thumb  is  always  opposable. 

Family  3.  Cercopithecidoe  (baboons,  mandrills  and  macaques). — The 
baboons  and  macaques  (Fig.  194,  C)  are  characterized  by:  quadrupedal 
habit  of  locomotion;  more  or  less  dog-like  heads;  ischial  or  rump 
callosities;  no  vermiform  appendix;  narrow  chests,  a  character  asso- 
ciated with  the  quadrupedal  habit;  very  large  canine  teeth;  cheek 
pockets.  They  are  omnivorous  in  diet,  as  are  the  other  Catarrhini. 
One  of  the  most  striking  characters  of  members  of  this  family  is  the 
brightness  of  their  coloring,  especially  that  of  nose,  cheeks,  and  rump. 
Bright  blue,  scarlet  and  lilac  colors  are  the  commonest  tints.  In 
habits  they  combine  those  of  the  arboreal  with  those  of  the  terrestrial 
types.  They  are  good  fighters  and  are  able  to  cope  with  many  of  the 
predaceous  terrestrial  animals  that  inhabit  the  Asiatic  and  African 
forests. 

Family  4-  Simiidce  (Anthropoid  Apes) . — The  members  of  this  family 
have  long  been  objects  of  especial  interest  on  account  of  their  close 
relationship  to  Man.  In  no  sense  are  they  to  be  thought  of  as  an- 
cestral to  Man;  rather  it  would  appear  that  they  are  " cousins,"  de- 


382  VERTEBRATE  ZOOLOGY 

rived  from  a  common  ancestral  stock.  Doubtless,  were  we  to  discover 
this  common  ancestor,  we  should  be  inclined  to  call  it  an  ape,  but 
it  certainly  was  not  very  much  like  any  of  the  present-day  apes. 

The  family  may  be  defined  as  follows:  tail  rudimentary  as  in  Man; 
no  cheek  pouches;  no  ischial  callosities  except  in  the  gibbon;  arms 
longer  than  legs;  the  great  toe  fully  opposable;  the  shoulders  broad; 
bipedal  habits;  always  a  vermiform  appendix;  hair  mainly  on  the 
ventral  side  of  the  body  and  on  the  limbs.  The  number  of  species  is 
not  great  and  there  is  so  general  an  interest  in  them  that  we  may 
spare  the  space  to  give  a  brief  description  of  the  principal  ones. 

The  gibbon  (Fig.  194,  D),  Hylobates,  is  a  rather  small  ape  with 
exceedingly  long  arms,  that  touch  the  ground  when  it  stands  erect. 
It  has  small  rump  callosities  similar  to  those  of  the  baboons.  Its 
dentition  is  adapted  for  a  fruit-eating  habit,  though  the  canines  are 
large  and  saber-like  for  self-defense.  The  skull  is  rounded  and  with- 
out the  sagittal  crest  characteristic  of  the  gorilla.  It  has  a  very 
erect  posture  both  in  walking  and  in  sitting,  the  head  being  set  upon 
the  neck  much  as  in  Man.  The  gibbon  has  a  tremendous  voice,  much 
more  voluminous  than  that  of  Caruso,  though  it  weighs  not  more 
than  about  sixty  pounds.  It  lives  in  heavily  wooded  mountain  slopes, 
remaining  largely  in  the  trees,  through  which  it  is  capable  of 
making  wonderful  speed.  With  its  long  arms  it  swings  along 
with  a  hand-stride  of  twenty  to  forty  feet,  and  never  misses  a  hold, 
though  it  must  calculate  the  distances  with  great  nicety  or  fall 
from  great  heights  to  the  ground.  Any  animal  that  can  use  its  arms 
and  hands  in  this  way  must  have  a  finely  developed  brain  back  of  it; 
indeed  the  gibbon's  brain  development  is  exceptional,  especially  in 
the  visual  and  tactual  centers.  When  walking  on  the  ground  the 
gibbon  walks  erectly  but  very  awkwardly,  balancing  itself  by  touch- 
ing the  knuckles  of  the  hands  to  the  ground.  It  is  evidently  about 
nine-tenths  an  arboreal  creature,  using  the  ground  only  when  trees 
are  not  available. 

The  orang  (Fig.  195),  Simia  satyrus,  is  a  large  ape  native  to 
Sumatra  and  Borneo.  It  is  short  and  stocky,  and  has  reddish  hair. 
Though  it  is  only  about  four  feet  in  height  it  has  an  arm-spread  of 
over  seven  feet.  The  head  is  short  and  broad  and  the  eyes  very  close 
together.  The  skull  has  a  sagittal  crest  for  the  attachment  of  the 
powerful  neck  muscles;  the  jaw  is  deep  and  massive  and  is  used  both 
for  tearing  open  fruits  and  in  fighting.  The  hands  are  the  chief 


MAMMALIA 


383 


weapons,  and  are  relied  upon  rather  than  the  teeth.  The  heavy 
weight  of  the  orang  makes  it  a  less  efficient  climber  than  is  the  gib- 
bon; and  its  mode  of  climbing  is  much  more  deliberate  and  man-like. 
It  builds  its  nest  in  trees  by  breaking  off  branches  and  arranging 


FIG.  195. — The  Orang-utang,  Simla  satyrus,  sitting  in  its  nest.    (From  Weysse, 
after  Shipley  and  McBride.) 

them  platform-fashion  in  the  crotch  where  two  large  limbs  meet. 
The  orang  appears  to  be  the  only  purely  herbivorous  member  of  the 
apes;  its  diet  consists  exclusively  of  fruits.  On  the  ground  it  runs 
on  all  fours  in  an  awkward  and  ineffective  way.  Its  intelligence 
has  been  experimentally  shown  to  be  greater  than  that  of  any  other 
creature  except  Man. 


384  VERTEBRATE  ZOOLOGY 

The  chimpanzee  (Fig.  194,  E  and  F),  Pan  pygmceus,  is  an  African 
ape  with  black  hair  and  a  height  of  about  five  feet;  it  is  less  bulky 
than  the  orang.  These  characters  make  the  chimpanzee  a  better 
climber  than  the  orang,  though  not  so  good  as  the  gibbon.  The  head 
is  larger  than  that  of  the  orang,  and  the  brow  ridges  are  very  prom- 
inent. There  is  a  pronounced  sagittal  crest  on  the  skull  for  the  attach- 
ment of  the  neck  musculature.  The  jaws  are  prognathous  and  re- 
semble those  of  prehistoric  Man.  It  builds  nests  much  like  those 
of  the  orang.  Some  authorities  distinguish  several  species  of  chim- 
panzees. They  are  largely  but  not  exclusively  fruit-eaters.  Their 
range  is  rather  limited,  being  confined  to  central  equatorial  Africa. 

The  gorilla  (Fig.  196),  Gorilla  gorilla,  is  much  the  largest  and 
fiercest  of  the  anthropoid  apes.  It  is  native  to  the  tropical  African 
forests  and  is  confined  to  a  very  restricted  territory.  It  stands  about 
five  feet  in  height,  but  is  so  massive  in  build  that  it  frequently  reaches 
a  weight  of  between  four  and  five  hundred  pounds.  If  it  had  legs 
in  proportion  to  its  arms  and  trunk  it  would  be  a  giant  of  at  least 
seven  feet  in  height.  The  gorilla  has  become  as  highly  specialized  as 
a  muscular  brute  as  has  Man  as  a  creature  of  intelligence  and  finesse. 
The  skull  has  a  much  heavier  sagittal  ridge  than  that  of  the  other 
apes,  and  this  is  accompanied  by  a  neck  musculature  of  tremendous 
strength.  The  jaws  are  prognathous  and  very  powerful,  with  large 
canine  teeth,  and  the  brow  ridges  are  very  prominent.  All  of  these 
characters  are  much  more  pronounced  in  the  old  males  than  in  the 
young  males  or  in  the  females;  a  condition  that  suggests  strongly 
their  highly  specialized  character.  The  gorilla  is  a  "negro"  ape  in 
the  sense  that  the  skin  is  black  and  the  hair  black  and  coarse.  In 
habits  the  gorilla  appears  to  be  transitional  between  the  arboreal  and 
the  terrestrial  types.  Both  hands  and  feet  approach  the  human  type, 
especially  in  young  specimens,  though  the  great  toe  remains  com- 
pletely opposable.  Gorillas  are  gregarious,  living  in  bands  of  con- 
siderable size,  with  an  old  male  at  the  head  of  each  band.  They  will 
not  run  from  Man  or  from  any  other  creature,  but  stand  their  ground 
and  put  up  a  ferocious  fight  with  both  hands  and  teeth.  The  state- 
ment has  often  been  made  that  the  gorilla  uses  sticks  or  clubs  in 
fighting,  but  this  has  never  been  confirmed  by  a  reliable  authority. 
From  the  purely  brute  physical  standpoint  the  anthropoids  have 
attained  a  higher  degree  of  specialization  than  any  other  primate,  but 
they  fall  far  short  of  Man  in  nervous  specialization. 


MAMMALIA 


385 


Family  5.  Hominidce  (Men) . — The  human  family  is,  structurally 
speaking,  very  closely  related  to  the  Simiidae;  in  fact  the  Simiidse 
and  the  Hominidse  are  more  closely  related  than  are  the  SimiidaD 


FIG.  196.— The  Gorilla,  Gorilla  gorilla.    (From  Lull,  after  mounted  specimen 
in  Philadelphia  Academy  of  Sciences.) 


386  VERTEBRATE  ZOOLOGY 

and  the  Cercopithecidae.  The  chief  differences  between  Man  and 
the  anthropoid  apes  are  viewed  as  the  direct  result  of  the  acquisition 
by  Man  of  terrestrial  habits,  erect  posture,  and  larger  brain,  all  of 
which  acquisitions  are  undoubtedly  closely  correlated.  These  pri- 
mary human  adaptations  are  accompanied  by  secondary  changes. 
Erect  posture,  for  example,  involves  a  series  of  adjustments,  such  as 
alterations  in  the  curvatures  of  the  spine,  changes  in  the  structure 
of  the  legs,  loss  of  grasping  power  of  the  great  toe,  and  increased 
length  of  legs.  The  following  comparison  between  Man  and  the 
anthropoid  apes  is  made  by  Gregory: 

"The  anthropoids  are  chiefly  frugivorous  and  typically  ar- 
boreal; when  upon  the  ground  they  run  poorly  and  (except  in 
the  case  of  the  gibbons)  use  the  fore  limbs  in  progressing;  Thus 
they  are  confined  to  forested  regions.  Man,  on  the  other  hand, 
is  omnivorous,  entirely  terrestrial,  erect,  bipedal  and  cursorial, 
an  inhabitant  primarily  of  open  country.  The  anthropoids  use 
their  powerful  canine  tusks  and  more  or  less  procumbent  incisors 
for  tearing  open  the  rough  rinds  of  large  fruits  and  for  fighting. 
Primitive  man,  on  the  contrary,  uses  his  small  canines  and  more 
erect  incisors  partly  for  tearing  off  the  flesh  of  animals,  which  he 
has  killed  in  the  chase  with  weapons  made  and  thrown  or  wielded 
by  human  hands.  These  implements  and  weapons  also  usually 
make  it  unnecessary  for  man  to  use  his  teeth  in  fighting  and  func- 
tionally they  compensate  for  the  reduced  and  more  or  less  de- 
fective development  of  his  dentition." 

Although  some  authors  recognize  four  species  of  Man,  the  best 
authorities  now  admit  of  but  a  single  species,  Homo  sapiens.  Possibly 
the  minor  divisions  are  the  equivalent  of  sub-species,  races,  or  va- 
rieties. Four  races  are  distinguished  by  Lull: 

Australian  race:  skull  long;  eyebrows  very  prominent;  teeth 
large,  especially  the  canines;  tall  and  long-limbed;  skin  brown;  hair 
black,  long  and  wooly.  Habitat:  Australia,  Dekkan,  Hindustan. 

Negroid  race:  skull  long;  forehead  round;  nasal  bones  flattened; 
teeth  sloping;  skin,  eyes,  and  hair  black;  hair  short  and  wooly. 
Habitat:  Madagascar  and  Africa  from  the  Sahara  desert  to  Cape  of 
Good  Hope. 

Mongolian  race:  skull  broad  and  short;  nose  flat;  eyes  small 
and  oblique;  stature  short  and  thick-set;  skin  golden  brown;  hair 
coarse,  straight  and  black;  beard  scanty.  Habitat:  east  of  a  line 


MAMMALIA 


387 


drawn  from  Lapland  to  Siam;  Chinese,  Tartars,  Japanese,  Malays, 
Esquimos,  North  and  South  American  Indians. 

Caucasian  race  is  usually  subdivided  into  three  varieties: 

A.  Mediterranean:  short;  slender;  long-headed;  with  hair  and  eyes 

dark  brown  to  black. 

B.  Alpine:  medium  height;  stocky  build;  round-headed;  hair  and 

eyes  dark  brown  or  black,  but  in  the  North  often  hazel  or 
gray,  probably  due  to  admixture  with  the  northern  varieties. 

C.  Nordic:  tall;  long-headed;  hair  flaxen,  red,  or  light  brown;  eyes 

blue,  gray,  or  green. 

Habitat  of  Caucasian  race:  mainly  Europe  and  North  America: 
includes  Moors,  Berbers,  Egyptians,  Kurds,  Persians,  Afghans? 
Hindus,  Turks,  Jews  and  Armenians. 

The  Immediate  Ancestors  of  Man 

According  to  Gregory,  Man  arose  from  an  early,  large-brained 
anthropoid  stock,  not  far  from  the  chimpanzee-gorilla  group.  Evi- 
dences point  toward  central  Asia  as  the  place  of  origin  and  early  de- 
velopment of  the  pre-human 
Hominidse.  The  time  of  origin 
is  believed  not  to  have  been 
later  than  early  Pliocene  and 
not  earlier  than  Miocene  times; 
thus  dating  back  some  hun- 
dreds of  thousands  of  years. 
The  earliest  fossil  remains  of 
the  Hominidse  consist  of  the 
relics  of  the  Java  "ape-man," 

Pithecanthropus  erectus  ^(Fig.  FlG  igy.—Skull  of  the  Java  ape-man, 
197).  Fragmentary  remains  of  Pithecanthropus  erectus.  (From  Lull,  after 
this  creature,  consisting  of  a  Dubois-) 

skull-cap,  a  thigh  bone,  and  two  upper  molar  teeth,  indicate  that  it 
was  intermediate  between  the  most  primitive  type  of  present-day  man 
and  the  highest  of  the  living  apes.  McGregor  has  reconstructed  busts 
of  Pithecanthropus,  of  the  most  primitive  of  extinct  human  species 
(Homo  neanderthalensis) ,  and  of  Homo  sapiens,  a  series  which  strik- 
ingly shows  the  gradual  evolution  away  from  apish  and  toward 
human  features  (Fig.  198). 


388 


VERTEBRATE  ZOOLOGY 


The  science  of  anthropology  concerns  itself  with  the  study  of  races 
of  man,  past  and  present,  a  field  that  cannot  be  more  than  touched 


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upon  in  a  volume  dealing  with  vertebrate  zoology.  Our  main  pur- 
pose has  been  to  place  man  in  his  appropriate  setting  among  his 
fellow  mammals. 


MAMMALIA  389 

SECTION  C.  UNGULATA  (HOOFED  MAMMALS) 

This  immense  assemblage  of  specialized  forms  includes :  two  orders 
of  extinct  mammals  already  dealt  with,  Condylarthra  and  Ambly- 
poda;  four  orders  of  present-day  terrestrial  mammals,  Artiodactyla, 
Perissodactyla,  Proboscidia,  and  Hyracoidea;  and  one  order  of  marine 
mammals,  Sirenia.  The  Ungulates  are  on  the  whole  the  most  highly 
specialized  of  terrestrial  mammals,  just  as  the  Cetacea  are  the  most 
highly  specialized  of  aquatic  mammals. 

ORDER  10.  ARTIODACTYLA  (EVEN-TOED  UNGULATES). — The  mam- 
mals of  this  group  are:  swine,  hippopotami,  peccaries,  camels,  deer, 
moose,  elk,  giraffes,  pronghorns,  cattle,  buffalos,  gnus,  antelopes, 
gazelles,  yaks,  sheep,  ibex,  goats,  and  many  other  less  well  known  types. 
It  is  a  major  assemblage  of  animals,  whose  size  on  the  average  is  large. 
They  are  purely  terrestrial,  though  some  of  them  are  mud-loving; 
for  the  most  part  they  are  cursorial,  though  some  are  heavy-bodied 
and  not  very  fleet  of  foot.  They  have  hoofs  on  two  or  four  toes.  The 
stomach  usually  has  several  chambers  in  adaptation  to  a  purely 
herbivorous  diet. 

Group  1.  Suina  (Swine-like  Ungulates). — This  group  consists  of 
three  families,  represented  respectively  by  the  hippopotamus,  swine 
proper,  and  peccaries.  The  hippopotamus  (Fig.  199,  A)  is  a  large 
heavy-bodied  aquatic  "hog,"  with  four  hoofs  on  each  foot.  It  is 
native  of  Africa,  as  is  also  the  pigmy  hippopotamus,  a  dwarf  species 
found  in  Liberia.  The  swine  proper  include  the  European  wild  hog 
(Fig.  199,  B),  the  wart  hog,  and  several  other  types.  The  domestic 
varieties  of  hog  have  been  derived  from  several  wild  species.  The 
peccaries  are  swift,  cursorial,  hog-like  creatures,  that  run  in  large 
packs,  and  on  account  of  their  sheer  numbers,  are  said  to  be  very 
dangerous  to  meet. 

Group  2.  Ruminantia  (Ruminants). — These  ungulates  "chew  their 
cud/'  by  which  is  meant  that  they  swallow  their  food  rapidly  and 
afterwards  regurgitate  it  into  the  mouth  for  further  mastication. 
Three  assemblages  of  these  forms  are  distinguished:  A,  Tragulina 
(Mouse  Deer);  B,  Tylopoda  (Camels,  Lamas);  C,  Pecora  (Deer, 
Antelopes,  Oxen,  Giraffes,  Goats  and  Sheep). 

The  chevrotanis  or  mouse  deer  are  intermediate  between  the  swine 
and  the  ruminants,  and  are  the  most  primitive  of  the  ruminants. 
The  camels  (Fig.  199,  C)  are  a  small  group  of  well-known  types,  con- 


390  ,     VERTEBRATE  ZOOLOGY 

fined  to  arid  regions  of  the  Old  World.  Camels  are  not  known  in 
the  wild  state;  all  are  domestic  or  feral.  The  ancestral  history  of 
the  camel  family  is  now  almost  as  well  worked  out  as  that  of  the 
horse.  Proverbial  for  the  camels  are  two  characters:  that  of  living 
for  long  periods  without  water,  and  the  use  of  the  fatty  humps  for 
food  when  compelled  to  fast.  Both  of  these  characters  may  be 
considered  as  adaptations  for  desert  life  and  have  made  it  a  highly 
valuable  beast  of  burden  and  transport  across  the  arid  trails  of  the 
Asiatic  and  African  deserts;  on  this  account  they  have  earned  the 
cognomen  "  ships  of  the  desert."  The  camel  is  very  valuable  for 
its  hair,  which  is  used  in  making  fabrics  highly  prized  for  their  rich- 
ness, softness,  and  wool-like  characters.  The  llamas  are  creatures 
with  camel-like  characters,  but  more  generalized  in  several  re- 
spects; they  might  be  called  the  camels  of  the  New  World,  for 
they  are  native  to  South  America.  They  are  of  value  chiefly  for 
their  rather  thin  hair,  which  is  coarser  than  that  of  the  camel 
and  is  the  material  out  of  which  are  made  vicuna  or  alpaca 
fabrics.  The  llama  has  the  disgusting  habit  when  irritated 
of  forcibly  spitting  the  contents  of  its  stomach  at  the  object  of  its 
annoyance. 

The  deer  family  is  a  very  large  one  and  includes  such  well-known 
types  as  elk,  moose,  reindeer,  etc.  They  are  characterized  by  the 
possession  of  antlers  in  the  male  sex,  and  in  the  reindeers  in  both 
sexes.  The  antlers  vary  in  degree  of  elaborateness  in  the  different 
genera,  ranging  from  the  small,  unbranched  horns,  as  in  Cervulus, 
to  the  complex  branching  antlers  of  the  elk  (Fig.  199,  D).  In  aU 
cases  they^are^solid  bony  strugjtaires,  as  opposed  to  the  hollow  horns 
of  the  Bovidse.  About  sixty  species  of  deer  are  known,  the  ma- 
jority of  which  are  Old  World  forms.  The  moose  is  the  king  of  the 
deer  family,  on  account  of  its  great  size  and  its  fighting  qualities. 
The  reindeer  is  the  most  northerly  of  the  deer,  occupying  circumpolar 
territory.  The  musk-deer  is  an  exceptional  type  in  that  it  has  no 
horns,  but  instead  is  possessed  of  long,  sharp  tusks,  probably  used 
in  digging  roots  for  food. 

The  giraffe  family  (Fig.  199,  E)  is  a  small  family  of  highly  spe- 
cialized ruminants  distinguished  by  their  great  height,  long  neck, 
and  slender  legs.  The  horns  differ  from  all  others  in  that  they  are 
merely  prominences  of  the  frontal  bones  of  the  skull  covered  with 
skin  and  hair.  Africa  is  the  home  of  the  giraffe,  as  well  as  that  of  the 


MAMMALIA 


391 


okapi,  a  small  less  specialized  member  of  the  giraffe  family,  which  is 
more  like  an  antelope  in  general  appearance. 

The  cattle  family  (Bovidae)  is  much  the  largest  family  of  rumi< 


FIG.  199. — Group  of  Artiodactyla  (Even-toed  Ungulates).  A,  Hippopotamus, 
Hippopotamus  amphibia;  B,  Wart  Hog,  Phocochoerus  cethiopicus;  C,  the  Bactrian 
Camel,  Camelus  bactrianus;  D,  Wapiti,  or  American  Elk,  Cervus  canadensis; 
E,  Giraffa  camelopardalis;  F,  Ibex,  Capra  swailica.  (Redrawn  and  somewhat 
modified;  A,  C,  after  Lydekker,  B,  E,  F,  after  Beddard,  D,  after  Fuertes.) 


392  VERTEBRATE  ZOOLOGY 

nants.  It  includes  oxen,  sheep,  goats  (Fig.  199,  F)  and  antelopes. 
The  most  prominent  distinguishing  character  of  the  group  is  the 
horns,  which  are  hollow  and  composed  of  chitin,  and  are  usually 
present  in  both  sexes.  A  large  number  of  the  Bovidse  have  been 
domesticated,  and  from  the  human  standpoint  are  the  most  impor- 
tant of  all  animals.  The  members  of  the  group  are  so  familiar  that 
no  description  of  the  different  species  is  necessary. 

ORDER  11.  PERISSODACTYLA  (ODD-TOED  UNGULATES). — In  this 
group  the  middle  digit  of  both  fore  and  hind  feet  is  preeminent  and 
carries  most  of  the  weight.  The  axis  of  the  limb  passes  through  the 
third  digit.  The  teeth  of  the  odd-toed  ungulates  are  usually  lophodont, 
a  type  characterized  by  the  presence  of  enamel  ridges  running  back 
and  forth  across  the  grinding  surface.  The  present-day  perissodactyls 
are  grouped  into  three  families:  Equidae,  Tapiridse,  and  Rhinocer- 
otidae. 

Family  1.  Equidce  (horses,  asses,  and  zebras). — The  members  of  the 
horse  family  (Fig.  200,  A)  are  characterized  by  the  possession  of  but 
a  single  functional  toe,  the  third  toe,  on  each  foot.  The  second  and 
fourth  toes  are  represented  by  vestigial  remnants,  called  "  splint 
bones."  The  molar  teeth  are  highly  complex  in  structure  and  wear 
down  through  most  of  the  life  of  the  individual,  so  that  the  age  of 
any  specimen  may  be  arrived  at  by  the  amount  of  wear  upon  the 
teeth.  All  of  the  modern  Equidae  are  placed  in  the  single  genus 
Equus.  Perhaps  the  most  convincing  record  of  the  ancestry  of  any 
vertebrate  group  is  that  of  the  horse.  With  respect  to  toes,  teeth, 
and  general  form,  the  gradual  perfection  of  the  present  highly  spe- 
cialized cursorial  type  may  be  traced  back  through  an  unbroken 
line  of  ancestors  to  a  very  generalized  ungulate  type  with  four  func- 
tional toes,  generalized  teeth,  and  comparatively  small  size.  The 
horse  has  played  and  is  still  playing  an  extremely  important  role  in 
the  progress  of  human  civilization.  Next  to  cattle  and  sheep  the 
horse  has  been  the  most  important  domesticated  animal;  but  if 
present  tendencies  furnish  a  reliable  criterion  of  the  future,  the  horse 
is  likely  to  be  displaced  by  the  motor-driven  vehicle. 

Family  2.  Tapiridce  (Tapirs)— The  tapirs  (Fig.  200,  B)  are  the 
most  generalized  of  modern  odd-toed  ungulates.  They  are  charac- 
terized by  moderate  size  and  by  a  short  proboscis  produced  by 
elongation  of  nose  and  upper  lip.  The  dentition  is  more  generalized 
than  that  of  the  horses,  there  being  forty-two  teeth,  a  number  very 


MAMMALIA 


393 


close  to  that  of  the  most  primitive  eutherian  mammals.  There  are 
four  toes  on  the  fore  feet  and  three  on  the  hind  feet.  The  tapirs  are 
confined  to  South  and  Central  America  and  to  the  Malay  Peninsula. 
Family  8.  Rhinocerotidce  (Rhinoceroses). — This  family  consists  of  a 
few  species  of  large,  massive  animals,  whose  general  appearance  is 
familiar  to  all  (Fig.  200,  C) .  They  are  distinguished  by  the  presence 


FIG.  200.— Group  of  Perissidactyla  (Odd-toed  Ungulates).  A,  Burchell's 
Zebra,  Equus  burchelli;  B,  American  Tapir,  Tapirus  terrestris;  C,  African  Rhinoc- 
eros, Rhinoceros  bicornis.  (Redrawn  and  modified:  A,  B,  after  Beddard;  C,  after 
Lydekker.) 

of  median  "horns"  on  the  nose;  but  the  structures  are  not  true  horns, 
being  composed  of  masses  of  agglutinated  hair  fastened  to  a  rough- 
ened patch  of  the  nasal  bones.  There  are  usually  three,  sometimes 
four,  toes  on  the  fore  feet,  but  in  either  case  the  third  toe  is  the  most 
important;  the  hind  feet  always  have  three  toes.  The  upper  lip  is 
long  and  more  or  less  prehensile,  but  not  elongated  into  a  proboscis 
as  in  the  tapirs.  The  skin  is  extremely  thick  and  the  hair  very  sparse. 
They  are  fierce  and  intractable,  charging  at  an  enemy  with  great 
fury  and  stopping  at  nothing.  Only  guns  of  large  caliber  and  hard- 


394  VERTEBRATE  ZOOLOGY: 

hitting  qualities  will  stop  their  mad  rush.  They  have  a  fairly  wide 
distribution,  being  native  to  both  India  and  Africa.  The  fossil  record 
of  the  ancestry  of  the  rhinoceros  is  almost  as  complete  as  that  of  the 
horse,  and  the  two  groups  appear  to  converge  upon  a  common  an- 
cestral group.  The  early  rhinoceroses  must  have  looked  more  like 
horses  than  the  present  forms,  which  have  grown  heavy  of  limb  and 
body  and  are  no  longer  typically  cursorial. 

ORDER  12.  PROBOSCIDIA  (ELEPHANTS). — This  group  comprises  the 
largest  and  in  many  respects  the  most  highly  specialized  of  terrestrial 
mammals.  They  are  characterized  by  the  elongation  of  the  nose  and 
upper  lip  into  a  very  long  trunk;  by  the  possession  of  five  functional 
digits  on  both  fore  and  hind  feet;  by  the  specialization  of  the  incisor 
teeth  of  the  upper  jaw  into  great  tusks;  and  by  the  extreme  type  of 
lophodont  molar  teeth.  The  skull  is  immensely  thick  and  the  bones 
contain  large  air  cavities;  there  is  no  clavicle;  the  cerebral  hemi- 
spheres are  much  convoluted,  but  they  do  not  cover  the  cerebellum; 
the  testes  are  abdominal  in  position. 

Elephants  walk  with  the  legs  stiff,  almost  as  if  they  were  jointless, 
an  adaptation  for  bearing  the  great  weight;  for  it  would  require  great 
muscular  effort  to  support  the  huge  bulk  of  these  animals  upon  a 
bent  type  of  limb.  Two  families  of  Proboscidia  are  distinguished: 
Elephantidce  and  Dinotheridce.  The  latter  were  Miocene  forms  char- 
acterized by  great  downwardly  directed  tusks  of  the  lower  jaw. 

There  are  but  two  living  species  of  elephant,  the  Indian  elephant 
(Fig.  201),  Elephas  indicus,  and  the  African  elephant  (Fig.  202),  E. 
africanus.  The  African  species  is  the  larger,  and  has  much  larger 
ears.  The  largest  specimen  on  record  is  probably  the  notorious 
"  Jumbo,"  which  was  about  eleven  feet  high  at  the  shoulder.  African 
elephants  are  wild  and  intractable  as  compared  with  their  Indian 
cousins;  and  therefore  are  seldom  seen  in  circus  parades.  The  Indian 
elephant  is  the  common  circus  elephant,  a  smaller  and  more  manage- 
able type.  In  its  native  country  it  is  used  extensively  as  an  equipage 
and  as  a  beast  of  burden.  As  a  species,  however,  they  are  not  de- 
pendable, some  being  vicious  and  others  perfectly  docile  in  disposi- 
tion. In  nature  they  are  creatures  of  the  jungle  and  are  purely 
herbivorous.  They  are  capable  of  defending  themselves  against  all 
enemies  except  Man.  An  encounter  between  an  elephant  and  a  tiger 
is  one  of  the  finest  gladiatorial  contests  that  the  world  affords. 

Elephants  have  been  credited  with  extraordinary  intelligence,  but 


MAMMALIA 


395 


they  are  much  less  wise  and  sagacious  than  they  have  been  painted. 
Undoubtedly  they  have  a  good  brain,  but  their  capacity  to  reason  is 


FIG.  201. — Indian  Elephant,  Elephans  indices.    (From  Weysse.) 


FIG.  202.— African  Elephant,  Elephans  africanvs.    (Redrawn  after  Beddard.) 
quite  rudimentary.    The  tenacity  of  their  memory  is  well  authenti- 
cated, for  they  have  been  known  to  cherish  an  injury  for  years.    In 


396  .     VERTEBRATE  ZOOLOGY 

all  probability  an  extraordinarily  keen  sense  of  smell  plays  a  prom- 
inent part  in  their  memory,  an  enemy  being  associated  with  a  special 
odor.  Even  in  human  beings,  whose  sense  of  smell  is  at  best  rudi- 
mentary, memories  of  all  sorts  are  inextricably  bound  up  with  odors. 

Elephants  live  to  a  great  age,  probably  in  the  neighborhood  of  two 
hundred  years.  In  this  connection  the  peculiar  arrangement  of  the 
molar  teeth  is  of  interest;  for  as  the  molar  teeth  that  first  emerge  are 
worn  off  by  long  years  of  use  other  molars  gradually  replace  them. 
The  grinding  teeth  are  arranged  as  though  in  the  arc  of  a  circle,  so 
that  only  two  or  at  most  three  on  each  jaw  are  in  contact  at  one  time. 
When  the  front  ones  wear  out  the  rest  move  up  and  take  their  places, 
until  in  very  old  animals  only  the  last  teeth  are  present.  This  denti- 
tion is  by  far  the  most  specialized  found  among  vertebrates. 

Among  the  best  known  recently  extinct  types  of  elephants  are 
the  mammoth  and  the  mastodon.  The  mammoth  was  more  nearly 
like  the  Indian  elephant  than  any  other  species,  but  was  much  larger. 
Its  tusks  were  enormous,  one  being  known  to  weigh  two  hundred  and 
fifty  pounds.  These  tusks  are  extremely  durable  as  is  demonstrated 
by  the  fact  that  much  of  the  ivory  now  in  use  in  the  form  of  billiard 
balls,  etc.,  has  been  made  from  them,  though  their  original  owners 
have  been  dead  for  thousands  of  years.  The  mastodon  was  about  as 
high  as  the  Indian  elephant,  seven  to  nine  feet,  but  was  much  more 
stockily  built  and  longer  bodied.  The  tusks  were  sometimes  as  much 
as  nine  feet  or  more  in  length. 

The  evolution  of  the  peculiar  characters  of  modern  elephants  is 
well  shown  in  a  series  of  extinct  forms,  as  represented  by  Lull  (Fig. 
203).  The  earliest  proboscidian  appears  to  have  been  a  form  like 
Moeritherium  (Fig.  203,  F'),  which,  though  rather  generalized  in  most 
respects,  shows  the  beginnings  of  elephantine  characters  in  the  air 
cells  in  the  back  of  the  skull,  in  the  enlarged  second  incisors  or  in- 
cipient tusks,  and  the  primitive  lophodont  molars.  It  was,  however, 
only  about  three  and  a  half  feet  high.  Transitional  stages  are  shown 
in  Palceomastodon  (Fig.  203,  E'),  in  Trilophodon  (Fig.  203,  D'),  and 
in  Stegodon  (Fig.  203,  C'),  in  which  all  of  these  characters  have 
approached  several  steps  nearer  the  present  condition,  as  shown  in 
upper  figure  (Fig.  203,  A'). 

ORDER  12.  SIRENIA  (DUGONGS  AND  MANATEES). — The  sirenians  are 
now  looked  upon  as  an  aquatic  offshoot  of  an  early  ungulate  stock 
distantly  related  to  the  proboscidians.  The  traditional  position  of 


MAMMALIA 


397 


these  aquatic  mammals    has   always  been  alongside  the  Cetacea 
(whales),  but  the  resemblances  between  these  two  aquatic  groups 


FIG.  203. — Evolution  of  head  and  molar  teeth  of  mastodons  and  elephants. 
A,  A',  Elephas,  Pleistocene;  B,  Stegodon,  Pliocene;  C,  C',  Mastodon,  Pleistocene; 
D,  D',  Trilophodon,  Miocene;  E,  E',  Palcp.omastodon,  Oligocene;  F,  F',  Mcerithwium, 
Eocene.  (From  Lull.) 


398 


VERTEBRATE  ZOOLOGY 


are  evidently  largely  homoplastic,  or  parallel  adaptations  to  a  similar 
habitat.  Both  dugongs  and  manatees  are  large,  almost  hairless  mam- 
mals, with  hind  limbs  absent,  and  with  the  tail  flattened  into  the 
semblance  of  a  caudal  fin  or  a  fluke.  The  nostrils  are  on  the  upper 
surface  of  the  snout;  there  are  no  clavicles;  the  stomach  is  complex 
and  resembles  that  of  the  un- 
gulates; the  testes  are  abdomi- 
nal in  position;  the  mammae 
are  pectoral  as  in  elephants. 

The  manatees  (Fig.  204) 
are  fairly  abundant  in  fresh 
waters  along  the  Atlantic 
coasts  of  North  America  and 
Africa.  They  are  said  to  be 
especially  numerous  among  the 
lagoons  of  the  Florida  Ever- 
glades. The  use  of  their  flesh 
as  meat  has  been  strongly 
urged;  for  they  feed  upon  noth- 
ing but  sea-weeds,  of  which 
there  is  an  inexhaustible  sup- 
ply. The  flesh  is  said  to  com- 
pare favorably  with  beef.  The 
manatees  have  but  six  cervical 

x^rtphrff*-    thprP    nrP    ««    rnnnv        FlG'  204.— Florida  Manatee,  Trichechus 

\ertebrse,  tnere  are  as  many  Mirostris    (Redrawn  after  Fuertes.) 
as  twenty  molar  teeth,  which 

seem  to  continue  to  increase  during  life.    In  these  two  respects  they 
are  unique  among  mammals. 

The  dugong  (Fig.  205),  Halicore,  is  an  oriental  and  Australian 
species,  with  whale-like  tail-flukes  instead  of  the  rhomboidal  type 
of  tail  paddle  seen  in  the  manatee.  It  is  more  extensively  specialized 
for  aquatic  life  than  the  manatee,  for  the  nostrils  are  more  dorsal, 
the  tail  is  more  fish-like  and  the  digits  have  no  claws.  It  is  said 
that  the  dugong  is  responsible  for  most  of  the  mermaid  legends,  for 
when  the  female  holds  her  young  to  her  pectoral  breast  by  means  of 
one  flipper  while  swimming  with  the  other,  she  presents  a  slightly 
human  resemblance. 

ORDER  13.  HYRACOIDEA  (CONEYS). — This  small  order  consists  of 
but  one  living  genus  of  primitive  ungulates.  The  coney  (Fig.  206) 


MAMMALIA 


399 


(Hyrax  or  Procavia)  bears  a  strong  resemblance  to  certain  rodents, 
the  short  ears  and  reduced  tail  being  especially  like  those  of  the  cavies. 


FIG.  205. — Dugong,  Halicore  dugong.     (Bedrawn  after  Lydekker.) 

They  are  unlike  the  ungulates  and  like  the  rodents  in  that  the  incisoi 
teeth  grow  from  persistent  pulps.  In  certain  other  respects  they  re- 
semble primitive  ungulates. 


y> 


FIG.  206. — Coneys  or  hyraces,  Hyrax  dbyssinicus.    (From  Lull,  after  Brehm.) 

Some  of  the  coneys  live  among  rocks,  while  others  are  partly  ar- 
boreal. The  Scriptures  describe  them  as  "exceeding  wise"  and  as 
"feeble  folk,"  but  the  observation  that  he  "cheweth  the  cud  but 


400  VERTEBRATE  ZOOLOGY 

divided  not  the  hoof"  is  without  foundation  on  either  count;  for 
they  are  not  ruminants,  and  there  are  four  hoofs  in  front  and  three 
behind. 

SECTION  D.  CETACEA  (WHALES  AND  DOLPHINS) 

This  assemblage  of  large  aquatic  mammals  is  profoundly  modified 
for  marine  life.  They  are  unquestionably  the  most  highly  specialized 
structurally  of  all  mammals,  although  certain  of  their  characters  are 
persistently  primitive.  In  older  classifications  they  have  usually 
been  placed  among  the  earlier  orders,  because  they  are  least  like 
Man,  who  was  looked  upon  as  the  ultimate  goal  of  organic  evolution. 
Is  it  too  serious  a  blow  to  human  complacency  to  have  to  cede  the 
honor  of  being  placed  at  the  top  of  the  systemic  ladder  to  the  whales? 
The  statement  that  the  whales  are  the  most  highly  specialized  mam- 
mals is  backed  up  by  the  following  criteria  of  specialization:  1,  the 
whales  are  farthest  removed  from  the  generalized  types  of  mammals  in 
all  of  their  adaptive  characters;  2,  they  have  undergone  losses  of  such 
typical  mammalian  structures  as  hair,  teeth  (in  some  groups),  claws, 
and  hind  limbs;  3,  the  skeleton  of  the  fore  limbs  is  progressively  spe- 
cialized by  the  addition  of  several  digits;  4,  they  have  reached  a  size 
unrivaled  in  the  world's  history,  far  surpassing  that  of  the  giant 
reptiles  of  Mesozoic  times;  5,  the  stomach  is  one  of  the  most  com- 
plex among  mammals;  6,  the  skull  of  some  of  the  whales  is  the  most 
asymmetrical  and  otherwise  specialized  among  mammals. 

Three  orders  of  Cetacea  are  distinguished:  Zeuglodontia  (extinct 
generalized  whales),  Odontoceti,  and  Mystacoceti. 

ORDER  14.  ODONTOCETI  (TOOTHED  WHALES). — This  order  includes 
the  sperm  whales,  narwhals,  beaked  whales,  porpoises  and  dolphins. 
They  are  characterized  by  the  presence  of  teeth  and  absence  of  whale- 
bone; by  the  possession  of  a  single  nostril  or  blow  hole;  by  asym- 
metry of  the  skull;  and  by  having  some  of  the  ribs  two  headed. 

The  sperm  whale  or  cachalot  (Fig.  207,  C),  Physeter,  is  probably 
the  largest  animal  that  ever  lived,  and  the  writer  was  fortunate  enough 
to  have  been  able  to  examine  and  to  record  the  measurements  of 
what  is  now  believed  to  have  been  the  largest  specimen  ever  authen- 
tically described.  This  was  the  well-known  Port  Arthur  whale,  that 
came  ashore  on  the  north  coast  of  the  Gulf  of  Mexico  in  March,  1910. 
This  animal  measured  on  a  straight  line  from  snout  to  end  of  flukes 
(not  following  curvatures  as  is  usually  done)  sixty-three  and  a  half 


MAMMALIA 


401 


FIG.  207. — Group  of  Cetacea.  A,  Killer  Whale,  Orca  gladiator  (after  True); 
B,  Common  Dolphin,  Delphinus  delphis  (after  Reinhardt);  C,  Sperm  Whale, 
Physeter  macrocephalus;  D,  Southern  Right  Whale,  Balcena  australis.  (C  and  D, 
after  Beddard.) 


402  VERTEBRATE  ZOOLOGY 

feet.  Its  circumference  in  front  of  the  flippers  was  thirty-seven  feet; 
it  was  twelve  feet  in  height  at  the  shoulders.  This  enormous  animal 
did  not  impress  one  as  a  long  slender  type,  but  as  distinctly  stocky, 
retaining  its  great  diameter  from  the  end  of  the  snout  to  within 
about  fifteen  feet  from  the  tail.  The  lower  jaw  of  the  sperm  whale 
is  long  and  narrow  and  is  armed  with  from  forty  to  forty-eight  conical 
teeth  that  fit  into  the  toothless  groove  of  the  upper  jaw.  A  large 
cavity  in  the  skull  is  filled  with  a  liquid  oil,  spermaceti,  which  is  a 
valuable  product.  This  reservoir  of  light  oil  is  believed  to  be  largely 
of  hydrostatic  value,  in  that  it  must  be  quite  buoyant.  The  huge 
skull  is  the  most  highly  modified  skull  known  for  a  mammal.  The 
right  maxillary  and  left  nasal  bones  are  much  larger  than  their  fel- 
lows, the  right  nasal  being  vestigial.  The  top  of  the  skull  has  a  great 
bony  crest  running  diagonally  instead  of  mesially  as  in  other  skulls. 
The  cervical  vertebrae  are  largely  fused  into  a  short  immovable  neck. 
The  sperm  whale  is  valuable  for  spermaceti,  for  oil  made  from  blub- 
ber, and  for  ambergris;  the  latter  is  a  very  valuable  product  said  to 
be  worth  its  weight  in  gold,  and  is  a  cumulative  byproduct  of  in- 
testinal digestion,  having  a  composition  somewhat  like  cholesterin. 
Ambergris  is  used  in  imparting  long-lasting  quality  to  fine  perfumes 
and  even  minute  quantities  add  value  to  considerable  volumes  of 
perfume.  The  food  of  the  sperm  whale  consists  largely  of  giant 
squids,  as  may  be  judged  by  the  remains  of  the  latter  found  in  the 
whale's  stomach. 

One  of  the  most  fish-like  of  the  toothed  whales  is  the  killer  (Fig. 
207,  A),  Orca,  a  small  species  that  has  the  reputation  of  killing  larger 
whales. 

Beaked  whales  are  animals  of  moderate  size,  seldom  more  than 
thirty  feet  in  length;  they  have  a  prolonged  muzzle  armed  with 
numerous  teeth.  They  are  quite  slender  and  doubtless  have  done 
duty  as  "sea  serpents."  Dolphins  and  porpoises  (Fig.  207,  B)  are 
small  whales  of  rather  generalized  structure.  They  have  teeth  in 
both  jaws,  and  the  head  is  more  mammal-like  than  that  of  other 
whales.  According  to  Flower,  there  are  nineteen  genera  of  these 
small  whales,  and  they  comprize  a  considerable  majority  of  all  exist- 
ing cetaceans.  They  are  distinctly  gregarious,  running  in  schools  of 
considerable  size.  Their  habit  of  leaping  out  of  the  water  at  inter- 
vals makes  them  an  interesting  sight  for  ocean  travelers.  Closely 
allied  to  the  porpoises  is  the  narwhal,  a  form  in  which  the  teeth  are 


MAMMALIA 


403 


reduced  to  a  single  tusk  in  the  upper  jaw,  which  protrudes  out  in 
front  like  a  spear.  This  tusk  is  twisted  in  structure  like  a  rawhide 
ox-whip  and  is  limited  to  the  males,  who  use  it  in  fencing  contests 
among  themselves. 

ORDER  15.  MYSTACOCETI  (WHALEBONE  WHALES). — The  whalebone 
or  baleen  whales  (Fig.  207,  D)  are  the  last  word  in  adaptive  specializa- 
tion among  mammals.  The  teeth  are  rudimentary  and  functionless, 
present  in  the  young  but  replaced  in  the  adult  by  baleen.  The  nostrils 
are  paired;  the  skull  is  symmetrical;  the  sternum  is  single;  the  ribs 
are  one  headed,  articulating  only  with  the  transverse  processes  of 
the  vertebrae.  The  group  is  composed  exclusively  of  large  forms, 
the  only  one  that  is  less  than  a  giant  being  the  pigmy  right  whale, 
which  is  only 
about  fifteen  feet 
in  length.  The 
baleen  or  whale- 
bone is  a  horny 
material  devel- 
oped from  the 
epithelial  lining 
of  the  mouth 
cavity.  It  is  dis- 
posed in  curtain- 
like  plates  (Fig. 
208),  frayed  out 

into  fringes  at  the  bottom.  The  plates  reach  a  length  of  twelve  or 
more  feet  and  are  triangular,  with  the  greatest  width  at  the  top  or 
attached  end.  As  many  as  three  hundred  and  seventy  blades  or 
curtains,  placed  with  their  edges  an  inch  or  so  apart,  have  been 
counted  in  a  single  mouth.  The  function  of  the  baleen  is  that  of  a 
strainer.  The  great  beast  rushes  through  the  water  with  the  mouth 
wide  open,  gathering  in  fishes  or  whatever  else  happens  to  be  in  the 
way.  Then  the  mouth  closes  and  the  water  is  forced  out  between 
the  sheets  of  baleen,  while  fishes,  etc.,  are  retained  and  swallowed. 
Such  huge  creatures  require  vast  quantities  of  food  and  cannot 
become  very  numerous.  Formerly  whalebone  was  a  commercial 
product  of  some  importance,  used  chiefly  as  stays  in  women's  gar- 
ments. Many  substitutes,  however,  have  been  discovered  and,  more- 
over, stays  have  gone  out  of  fashion;  so  that  the  market  value  of  the 


FIG.  208.— Skull  of  Baleen  Whale,   Balcbna   mysticetus. 
(From  Weysse,  after  Glaus  and  Sedgwick.) 


404  VERTEBRATE  ZOOLOGY 

commodity  has  been  greatly  depressed.  A  single  large  whale  pro- 
duces several  tons  of  whalebone,  and,  since  a  ton  used  to  be  worth 
about  ten  thousand  dollars,  the  capture  of  a  single  baleen  whale 
meant  a  small  fortune  to  the  whaler. 

Rorquals  are  a  type  of  whalebone  whale  with  comparatively 
small  heads,  a  distinct  dorsal  fin,  and  with  a  throat  deeply  corrugated 
into  longitudinal  furrows.  The  flipper  has  only  four  fingers,  but 
each  finger  is  very  long,  having  many  extra  joints.  They  range 
in  length  from  forty  to  nearly  seventy  feet;  one  species  has  a 
record  of  eighty-five  feet  in  length.  •  The  cervical  vertebrae  are  all 
separate. 

Right  Whales  are  the  more  typical  baleen  whales.  They  have 
no  dorsal  fin;  the  head  is  very  large,  being  about  one-fourth  of  the 
entire  length;  the  baleen  is  very  long;  the  throat  is  not  corrugated; 
the  cervical  vertebrae  are  fused  into  a  solid  mass.  The  Greenland 
right  whale  is,  perhaps,  the  best  known  of  all  whales.  It  has  a  very 
limited  distribution,  being  confined  to  the  Arctic  Ocean.  It  grows 
to  be  about  seventy  feet  in  length.  The  pursuit  of  whaling  used  to 
be  one  of  the  most  romantic  and  dangerous  of  human  occupations; 
but  with  the  advent  of  whaling  guns,  with  which  the  great  creatures 
may  be  harpooned  at  a  safe  distance,  the  danger  is  largely  eliminated, 
though  much  of  the  romance  remains.  The  southern  right  whale,  a 
close  relative  of  the  Greenland  species,  has.  a  wide  range,  avoiding 
only  the  Arctic  regions.  The  two  species  never  occur  in  the  same 
territory.  It  is  less  prized  by  the  whaler  on  account  of  the  relatively 
short  and  coarse  whalebone. 

Whales  as  a  whole  are  much  less  numerous  than  they  were  a  cen- 
tury ago  and  it  seems  probable  that,  unless  some  protection  is  given 
them,  they  are  likely  to  become  extinct  before  another  century  rolls 
by.  Man  seems  to  have  no  compunctions  in  his  lust  for  commercial 
profit,  and  even  these  noble  creatures  of  the  deep  may  soon  go  the 
ways  of  the  giants  of  ages  past. 

THE  DEVELOPMENT  OF  MAMMALS 

It  is  much  more  difficult  to  give  a  concise  account  of  development 
of  mammals  than  of  any  other  of  the  vertebrate  classes,  because 
there  is  such  a  wide  range  of  diversity  of  conditions.  In  the  first 
place  it  will  be  recalled  that  some  of  the  mammals  lay  large  eggs  es- 
sentially as  in  reptiles,  that  others  have  a  sort  of  uterine  gestation 


MAMMALIA 


405 


without  establishing  any  definite  structural  connection  between  the 
fetal  and  the  uterine  membranes,  and  that  still  others  have  a  well- 
defined  type  of  placental  gestation.  We  may  quickly  dispose  of  the 
situation  involved  in  the  egg-laying  mammals  by  saying  that  their 
mode  of  development  is  essentially  sauropsidan,  and  need  not  be 
repeated  here. 

The  marsupials  present  a  wide  variety  of  conditions.  Their  eggs 
though  minute  are  somewhat  larger  than  those  of  the  monodelphian 
mammals.  The  embryo  has  a  brief  period  of  uterine  gestation, 
though  no  fixed  nor  definite  uterine  attachment  is  established.  In 


COfl 


"a/I 


FIG.  209. — Diagram  of  the  embryo  and  fetal  membranes  of  the  marsupial 
Uypsiprymnus  rufescens  (on  the  left),  all,  allantoic  cavity;  amn,  amnion;  amn.  c, 
amniotic  cavity;  coel,  extra-embryonic  coelom;  ser,  chorion  or  serous  membrane. 
(From  Parker  and  Haswell,  after  Semon.) 

FIG.  210. — Diagram  of  embryo  and  fetal  membranes  of  Phascolarctus  cinereus 
(on  the  right).  Letters  as  in  fig.  209.  (From  Parker  and  Haswell,  after  Semon.) 

most  marsupials  a  large  part  of  the  surface  of  the  egg  is  covered  over 
by  the  compressed  and  expanded  yolk-sac,  as  in  Hypsiprymnus 
(Fig.  209).  In  Phascolarctus  (Fig.  210)  the  allantois  is  in  contact 
with  part  of  the  surface.  In  Perameles  (Fig.  211)  a  primitive  type 
of  allantoic  placenta  is  formed  and  sends  out  vascular  outgrowths 
into  the  maternal  tissues,  much  as  does  the  Trager  or  primary  pla- 
centa of  the  rodents  and  armadillos,  among  true  placental  mammals. 
Just  here  it  may  not  be  out  of  place  to  recall  that  it  is  believed  by 
several  leading  authorities  that  the  conditions  found  in  the  mar- 
supials of  to-day  are  not  primitive  but  largely  degenerate,  and  that 


406 


VERTEBRATE  ZOOLOGY 


COfl 


Perameles  with  its  primitive  placenta  represents  a  more  nearly  prim- 
itive condition  than  any  other  living  marsupial  so  far  studied.  Such 
a  view  would  involve  the  corollary  that  both  modern  marsupials 
and  modern  placental  mammals  have  been  derived  from  a  primitive 
placental  ancestry,  possibly  akin  to  the  insectivores. 

Conditions  in  Placental  Mammals. — Some  of  the  simpler  types, 
such  as  that  of  the  pig  and  the  horse,  are  not  unlike  those  seen  in 
the  marsupial,  Perameles,  but  in  others,  as  for  example  the  primates, 

the  armadillos,  etc.,  the  con- 
ditions are  very  much  modi- 
fied. In  earlier  stages,  how- 
ever, the  differences  are 
slight. 

The  egg  of  the  placental 
mammal  is  extremely  small 
and  essentially  yolkless,  yet 
many  changes  take  place 
that  seem  to  occur  with  refer- 
ence to  a  large  yolk  supply. 
The  embryo  is  developed 
from  a  small  region  of  the 
blastula,  and  is  cut  off  from 
the  extra-embryonic  area, 
with  which  it  remains  con- 
nected by  a  slender  yolk- 
stalk.  There  is  a  fairly  large 
yolk-sac,  without  any  yolk 
content,  upon  which  a  vitel- 
line  circulation  develops  up -to  the  point  of  blood  formation  and 
then  goes  no  further.  Amnion,  chorion,  and  allantois  form  much  as 
in  birds,  though  secondary  modifications  of  all  of  these  membranes 
are  found  in  various  groups.  All  of  these  conditions  seem  to  admit 
of  but  one  interpretation:  that  the  small  yolkless  mammalian  ovum 
is  the  lineal  descendant  of  a  large-yolked  egg  similar  to  that  of  the 
monotremes  or  the  reptiles,  and  that  the  yolk  has  been  lost  in  con- 
nection with  the  habit  of  uterine  gestation.  With  all  the  conserva- 
tiveness  of  the  typical  germ  cell,  the  mammal  egg  persists  in  behav- 
ing much  as  though  it  had  a  large  supply  of  yolk  upon  which  it  had 
to  depend  for  nourishing  the  embryo. 


FIG.  211. — Diagram  of  the  embryo  and 
placenta  of  the  marsupial  Perameles  obe- 
sula.  Letters  as  in  fig.  209.  In  addition 
— all.  s,  allantoic  stock;  mes,  mesenchyme 
of  outer  surface  of  allantois  fused  with 
mesenchyme  of  serous  or  chorionic  mem- 
brane; st,  sinus  terminalis;  ut,  uterine  wall. 
(From  Parker  and  Has  well,  after  Hill.) 


MAMMALIA 


407 


Cleavage  and  Early  Development  in  a  Placenta!  Mammal. — It  is 

not  easy  to  compare  the  cleavage  (Fig.  212)  of  the  mammalian  ovum 
with  that  of  any  other  form.  It  appears  deceptively  simple,  but 
we  know  that  this  apparent  simplicity  is  a  camouflage,  for  subse- 


FJG.  212. — Cleavage  of  the  ovum  of  the  rabbit.  A,  Two-cell  stage,  24  hours 
after  coitus,  showing  the  two  polar  bodies  separated.  B,  Four-cell  stage,  25% 
hours  after  coitus.  C,  Eight-cell  stage,  a,  albuminous  layer  derived  from  the 
wall  of  the  oviduct;  z,  zona  radiata.  (From  Kellicott,  after  Assheton.) 

quent  events  reveal  that  the  apparent  holoblastic  cleavage  gives 
results  that  are  similar  to  those  resulting  from  a  sauropsidan  type 
of  meroblastic  cleavage.  It  appears  that  the  first  two  cleavages  are 
total  and  equal,  just  as  in  Amphioxus.  After  that  the  cleavages 


FIG.  213. — Morula  and  early  blastodermic  vesicles  of  the  rabbit.  The  zona 
radiata  and  albuminous  layer  are  not  shown.  A,  Section  through  a  morula 
stage,  47  hours  after  coitus.  B,  Section  through  very  young  vesicle,  80  hours 
after  coitus.  C,  Section  through  more  advanced  vesicle,  83  hours  after  coitus; 
taken  from  uterus,  c,  cavity  of  blastodermic  vesicle;  i,  inner  cell  mass;  w,  wall 
of  the  blastodermic  vesicle  (trophoblast).  (From  Kellicott,  after  Assheton.) 

are  not  easy  to  follow,  since  the  cells  seem  to  shift  about  and  not  to 
retain  their  original  positions. 

The  blastula  stage  takes  the  form  of  a  solid  mass  of  cells,  the 
morula  (Fig.  213,  A),  in  which  a  peripheral  layer  of  cells,  the  tropho- 


408  VERTEBRATE  ZOOLOGY 

blast,  is  distinguished  from  the  inner-cell-mass.  Subsequently  (Fig. 
213,  B  and  C)  the  trophoblast  separates  from  the  inner-cell-mass 
except  at  the  animal  pole  and  a  large  cavity  filled  with  fluid  appears 
between  the  two  layers.  The  trophoblast  layer  is  a  temporary  struc- 
ture serving  as  a  sort  of  primitive  placenta  for  the  young  embryo 
and  helping  the  latter  to  gain  its  first  connection  with  the  uterine 
membrane.  A  specialized  region  of  the  trophoblast,  called  the 
Trager,  sends  short  papillae  into  the  uterine  mucosa,  opening  the 
way  for  the  true  placental  villi  that  come  later.  The  inner-cell-mass 
forms  the  entire  embryo,  together  with  the  embryonic  membranes, 

amnion,  chorion,  allantois,  and 
-fcm  yolk-sac.  At  first  a  round  ball 
of  cells,  the  inner-cell-mass  flat- 
tens out  to  form  a  thin  lens- 
shaped  mass  in  contact  with  the 
attached  part  of  the  trophoblast, 
or  Trager.  Later  two  layers  form, 
ectoderm  and  endoderm,  by  a 
sorting  out  of  two  types  of  cells, 
or  a  migration  inwards  of  the 

'    h  ^^  stf^   zv      endoderm  cells.     This  process  is 

^**  /^  the  equivalent  of  the  first  step  in 

FIG.  214.-Sectio7^ough  the  fully  gastrulation,  but  cannot  readily 
formed  blastodermic  vesicle  of  the  rab-  be  compared  with  the  equivalent 
bit.  fcm,  granular  cells  of  inner  cell  mass;  process  in  any  other  type  of  em- 
Iroph.  trophoblast;  z.  p.  zona  pellucida  *  ^ 

(Froi  Kellicott,  after  Quain.)  b^°-   Once  the  two-layered  germ- 

inal  disk,  early  gastrula,  is  formed, 

the  remainder  of  the  process  of  embryogenesis  is  much  like  that  of 
the  Sauropsida  and  need  not  be  further-  described. 

The  development  of  the  embryonic  membranes,  however,  differs 
in  many  ways  from  that  seen  in  the  bird.  The  layer  of  endoderm, 
at  first  confined  to  the  upper  part  of  the  vesicle,  spreads  until  it  forms 
a  complete  inner  lining  for  the  trophoblast.  The  g;ut  of  the  embryo 
is  pinched  off  from  the  upper  part,  leaving  an  empty  yolk-sac  below, 
connected  with  the  gut-endoderm  by  a  slender  yolk-stalk.  The 
amnion  sometimes  forms  as  in  the  chick  (Fig.  216),  by  a  fold  of  the 
somatopleure,  which  also  produces  the  outer  layer  or  chorion;  but 
sometimes  the  amnion  forms  by  means  of  a  cavity  opening  up  in 
the  midst  of  the  ectodermic  mass,  a  short-cut  method  used  by  the 


MAMMALIA  409 

insectivores  (Fig.  215),  rodents,  armadillos  and  man.  The  allantois 
forms  as  in  birds,  but  frequently  remains  rudimentary  as  in  man 
(Fig.  217);  but  in  some  cases,  as  in  the  rabbit  (Fig.  216),  it  forms  a 
primitive  type  of  allantoic  placenta  much  like  that  seen  in  the  mar- 
supial, Perameles. 

The  formation  of  the  true  chorionic  placenta  is  a  complicated 
process.  The  mesodermic  layer  of  the  chorion,  which  becomes  highly 
vascular,  and  becomes  connected  with  the  embryonic  circulation, 
sends  out  branching  processes,  chorionic  villi,  into  the  uterine  tissues, 
which  penetrate  the  uterine  lymph  cavities  and  absorb  liquid  nutri- 


FIG.  215. — Diagram  of  the  formation  of  the  amnion  in  the  Insectivores.  Black; 
embryonic  ectoderm;  heavy  stipples,  trophoblast;  light  stipples,  endoderm, 
oblique  ruling,  mesoderm.  A,  before  the  appearance  of  the  amniotic  cavity; 
inner  cell  mass  differentiated  into  ectoderm  and  mesoderm;  endoderm  extending 
completely  around  the  wall  of  the  vesicle.  B,  The  amniotic  cavity  (a)  appearing 
in  the  ectoderm.  C,  Enlargement  of  the  amniotic  cavity.  Mesoderm  expanded 
and  split  into  somatic  and  splanchnic  layers,  separated  by  the  ccelom.  s,  prim- 
itive streak.  (From  Kellicott,  after  Keibel.) 

ment  directly  from  the  maternal  supply.  The  maternal  tissues  be- 
come thick  and  congested  in  these  regions,  and  the  fetal  and  maternal 
tissues  together  constitute  the  definitive  placenta.  The  entire 
chorion  is  at  first  provided  with  simple  villi,  but  later  only  certain 
regions  retain  the  villi  and  act  as  placental  areas.  Frequently  the 
placental  area  is  discoidal  in  shape,  as  in  the  primates,  in  some  of 
the  edentates,  and  in  many  of  the  rodents;  sometimes  the  placental 
area  is  band-like  or  zonary,  as  in  the  carnivores;  and  in  the  case  of 
the  ungulates  it  is  cotyledenous,  in  which  case  thick  knots  of  villi 
are  scattered  over  almost  the  entire  chorion,  separated  by  extensive 
non-villous  areas. 

Parturition  or  birth  takes  place  at  widely  different  stages  of  matu- 


410 


VERTEBRATE  ZOOLOGY 


rity  in  the  different  mammalian  groups.    In  some  species,  as  in  cattle 
and  horses,  the  young  at  birth  are  well  advanced  and,  within  a  few 


h      ta 


vb 


FIG.  216. — Diagram  of  the  formation  of  the  embryonic  membranes  and  append- 
ages of  the  rabbit.  A,  at  the  end  of  the  ninth  day;  B,  early  the  tenth  day;  C,  at 
end  of  tenth  day.  Ectoderm,  black;  endoderm,  dotted;  mesoderm,  gray,  al,  al- 
lantois;  as,  allantoic  stalk;  6,  tail  bud;  c,  heart;  d,  trophoderm;  e,  endoderm; 
ex,  exoccelom;  /,  foregut;  h,  hind-gut;  m,  mesoderm;  N,  central  nervous  system; 
p,  pericardial  cavity;  pa,  proamnion;  s,  marginal  sinus  (sinus  terminalis);  t,  tropho- 
blast;  ta,  tail-fold  of  amnion;  v,  trophodermal  villi;  vb,  trophoblastic  villi;  y,  cav- 
ity of  yolk-sac;  y.  s,  yolk-stalk.  (From  Kellicott,  after  Van  Beneden  and  Julin.) 


MAMMALIA 


411 


hours  after  birth,  are  able  to  walk  or  even  to  run,  and  require  little 
parental  care  except  in  connection  with  mammary  feeding.  In  other 
species,  as  in  the  carnivores  and  rodents,  the  young  are  born  naked, 


FIG.  217. — Diagram  illustrating  the  formation  of  the  umbilical  cord  and  the 
relations  of  the  allantois  and  yolk-sac  in  human  embryo.  The  heavy  black  line 
represents  the  embryonic  ectoderm;  the  dotted  line  marks  the  line  of  transition  of 
the  body  (embryonic)  ectoderm  and  that  of  the  amnion.  Stippled  areas,  meso- 
derm.  Ac,  Amniotic  cavity;  Al,  allantoic  cavity;  Al,  allantois;  Be,  exoccelom; 
Bs,  body  stalk;  Ch,  chorion;  P,  placenta;  Uc,  umbilical  cord;  V,  chorionic  (tro- 
phodermic)  villi;  Fs,  yolk-sac.  (From  Kellicott,  after  McMurrich.) 

blind,  and  helpless  and  need  much  care  for  a  considerable  period. 
The  human  infant,  while  not  as  immature  as  some  of  those  just 
mentioned,  is  decidedly  helpless  and  needs  care  longer  than  any 
other  creature. 


PARTIAL  LIST  OF  LITERATURE 

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Bensley,  B.  A.:  On  the  Evolution  of  the  Australian  Marsupialia,  with  Re- 
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Boulenger,  G.  A.:  Teleostei;  Cambridge  Natural  History,  Vol.  VII,  1904. 

Bourne,  G.  C.:  An  Introduction  to  the  Study  of  the  Comparative  Anatomy 
of  Animals,  2  vols.,  1900. 

Brehm,  A.  E.:  Tierleben;  allgemeine  Kunde  des  Tierreichs.    4th  edition,  re- 
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Bridge,  T.  W.:  Fishes,  Cambridge  Nat.  Hist.,  Vol.  VII,  1904. 

Bronn,  H.  G.:  Klassen  und  Ordnungen  des  Thierreichs. 

Caldwell,  H. :  Embryology  of  Monotremata  and  Marsupialia,  1887. 

Case,  E.  C.:  The  Permo-Carboniferous  Red  Beds  of  North  America  and  their 
Vertebrate  Fauna,  1915. 

Cerfontaine,  P.:  Recherches  sur  la  developpement  de  1'Amphioxus,  1906. 

Chamberlin,  T.  C.:  On  the  Habitat  of  the  Early  Vertebrates,  1900. 

Child,  C.  M.:  Individuality  in  Organisms,  1915. 

Child,  C.  M.:  Senescence  and  Rejuvenescence,  1915. 

Cope,  E.  D. :  Batrachia  of  North  America. 

Cope,  E.  D.:  Article  on  Amphibia  in  Riverside  Natural  History. 

Cope,  E.  D. :  Crocodiles,  Lizards,  and  Snakes  of  North  America,  1900. 

Coues,  E. :  Key  to  North  American  Birds,  1887. 

Dean,  B.:  Fishes  Living  and  Fossil,  1895. 

Dean,  B.:  Fin-fold  Origin  of  Paired  Fins,  1896. 

Dean,  B.:  Development  of  Garpike  and  Sturgeon,  1895. 

Dean,  B. :  Chimeroid  Fishes  and  Their  Development,  1906. 

Dendy,  A. :  Outlines  of  Evolutionary  Biology,  1912. 

Dickerson,  M.  C.:  The  Frog  Book,  1907. 

Ditmars,  R.:  The  Reptile  Book,  1907. 

Duckworth,  W.  L.  H.:  Prehistoric  Man,  1912. 

Evans,  A.  H.:  Birds;  Cambridge  Natural  History,  Vol.  IX,  1909. 

Flower,  W.  H.:  Osteology  of  Mammals,  1885. 

Flower,  W.  H.,  and  Lydekker,  R.    Mammals,  1891. 

Gadow,  H.:  Amphibia  and  Reptiles;  Cambridge  Natural  History,  Vol.  VIII, 
1901. 

413    * 


414  PARTIAL  LIST  OF  LITERATURE 

Gadow,  H.:  Volume  on  Birds  in  Bronn's  Tierreich. 

Gegenbaur,  C.:  Vergleichende  Anatomic  der  Wirbeltiere,  2  vols.  1898  and 

1901. 

Goode  and  Bean. :  Oceanic  Ichthyology,  1895. 

Goodrich,  E.  S.:  Cyclostomes  and  Fishes;  Lankester's  Zoology,  Vol.  IX,  1909. 
Gregory,  W.  K.:  Present  Status  of  the  Origin  of  the  Tetrapoda,  1915. 
Gregory,  W.  K.:  Studies  on  the  Evolution  of  the  Primates,  1916. 
Gregory,  W.  K.:  Theories  of  the  Origin  of  Birds,  1916. 
Giinther,  A.:  Introduction  to  the  Study  of  Fishes,  1880. 
Harmer,  S.  F.:  Hemichordata;  Cambridge  Natural  History,  Vol.  VII,  1904. 
Hatschek,  B.:  The  Amphioxus  and  its  Development;  English  Translation, 

1893. 

Hay,  O.  P. :  The  Batrachians  and  Reptiles  of  the  State  of  Indiana,  1892? 
Hegner,  R.  W.:  College  Zoology,  1912. 
Herdm&n,  W.  A.:  Ascidians  and  Amphioxus;  Cambridge  Natural  History, 

Vol.  VII,  1910. 

Hertwig,  0.:  Manual  of  Zoology,  1910. 
Holmes,  S.  J.:  The  Biology  of  the  Frog,  1914. 

Huxley,  T.  H.:  Manual  of  the  Anatomy  of  Vertebrated  Animals,  1872. 
Jordan,  D.  S.:  Guide  to  the  Study  of  Fishes,  1905. 
Jordan,  D.  S.:  Fishes,  1907. 

Jordan  D.  S.,  and  Evermann,  B.  W.:  Fishes  of  North  America,  4  vols.  1900. 
Jordan,  D.  S.,  and  Evermann,  B.  W. :  The  Aquatic  Resources  of  the  Hawaiian 

Islands.    Part.  I,  The  Shore  Fishes;  Bui.  U.  S.  Fish  Com.,  Vol.  XXIII, 

1905. 

Jordan,  D.  S.,  and  Kellogg,  V.  L. :  Evolution  and  Animal  Life,  1908. 
Keibel,  F.,  and  Mall,  F.  P.:  Manual  of  Human  Embryology,  1910. 
Kellicott,  W.  E.:  Outlines  of  Chordate  Development,  1913. 
Kingsley,  J.  S.:  Textbook  of  Vertebrate  Zoology,  1899. 
Kingsley,  J.  S. :  Systematic  Position  of  the  Caecilians,  1902. 
Kingsley,  J.  S.:  Comparative  Anatomy  of  Vertebrates,  2nd  ed.,  1917. 
Knowlton,  F.  H.:  Birds  of  the  World,  1909. 
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Goodrich),  1909. 

Lydekker,  R. :  The  New  Natural  History. 
Lillie,  F.  R.:  The  Development  of  the  Chick,  1908. 

Locy,  W.  A. :  Contribution  to  the  Structure  and  Development  of  the  Verte- 
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Lucas,  F.  A.:  The  Beginnings  of  Flight,  1916. 
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Marshall,  A.  M.:  The  Frog:  an  Introduction  to  Anatomy,  1896. 


PARTIAL  LIST  OF  LITERATURE  415 

Matthew,  W.  D.:  The  Arboreal  Ancestry  of  the  Mammals,  1904. 

Matthew,  W.  D.:  Climate  and  Evolution,  1915. 

Minot,  C.  S. :  Human  Embryology,  1892. 

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Newman,  H.  H.,  and  Patterson,  J.  T.:  Development  of  the  Nine-banded 
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Osborn,  H.  F.:  The  Age  of  Mammals,  1910. 

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Osborn,  H.  F.:  The  Origin  and  Evolution  of  Life,  1918. 

Parker  and  Haswell:  A  Text-book  of  Zoology,  1910. 

Patten,  W.:  The  Evolution  of  the  Vertebrates  and  Their  Kin,  1912. 

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Blindwiihle  Ichthyophis,  1890. 

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Walter,  H.  E.:  The  Human  Skeleton,  1918. 

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Weysse,  A.  W.:  Synoptic  Text-book  of  Zoology,  1909. 

Wiedersheim,  R.:  Comparative  Anatomy  of  Vertebrates  (English  transla- 
tion), 1907. 

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INDEX 


(Asterisks  indicate  pages  on  which  occur  illustrations  of  the  subject 

mentioned) 


Aard  Vark,  375  *,  377,  378 

Acanthiuridse,  146 

Acanthodei,  110 

Acanthodidae,  110 

Acanthopterygii,  98,  110,  111,  146-150 

Accessory  respiratory  organs  of  fishes, 
108,  109 

Acipenser,  133;  eggs  of,  164;  A.  ruthenus 
132* 

Acris  gryllus,  201 

Acrobates,  357 

Adaptations,  laws  of,  16-20;  aquatic, 
16,  17  *;  climbing  (scansorial),  18  *; 
cursorial,  18;  fossorial,  18 

Adaptive  changes  incident  to  ter- 
restrial life,  175,  176 

Adaptive  radiation,  19;  of  reptiles, 
211  *;  of  snakes,  258,  259;  of  mar- 
supials, 353 

Adelochorda  (see  Cephalochordata) 

Adobenus  obesus,  369  * 

Muropus,  368 

^Bpyornithes,  287 

Aglossa,  196-198 

Agnathostomata,  69 

Agouti,  372 

Air-bladder,  of  Teleostomi,  128;  of 
Polypterus,  131 

Albatross,  293  *,  294,  295 

Alcedo,  foot  of,  280  * 

Alecteromorphae,  289 

Alimentary  system,  of  Amphioxus,  38; 
of  tunicate,  51;  of  Balanoglossus,  62; 
in  Squalus,  114;  of  fishes,  101;  of 
turtle,  230,  231;  of  bird,  267 

Allantoida,  68,  69 

Allantois,  68,  69;  in  reptiles,  212,  213  *; 
in  birds,  320,  321  *;  in  placenta! 
mammals,  409,  410*,  411*;  in 
Man,  411  * 

Allen,  B.  W.,  195 


Alligator  Gar,  133,  134 

Alligator  mississippiensis,  245  * 

Allotheria,  338 

Alopecias  vulpes,  122  * 

Alosa,  eggs  of,  164 

Altrices,  323 

Alytes  obstetricans,  197  *,  198 

Ambergris,  402 

Amblypoda,  340,  342-344 

Amblyrhynchus  cristatus,  250  *,  252 

Amblystoma  tigrinum,  189,  190  *,  192 

Amia,  109, 133, 167;  A.  calva,  133 

Amiidse,  133  v 

Ammocoetes  larva,  38  f\  5 

Amnion,  68,  69;  in  reptiles,  212,  213  *; 

in  birds,  319,  320  *,  321;  in  placenta! 

mammals,  409  *,  410  *,  411  * 
Amniota,  68,  69,  95  *,  96 
Amphibamus,  180  * 
Amphibia,  13,  33,  68,  69;  present  and 

past  status,  173,  174,  origin  of,  175, 

176;  characters  of,  183,  184; 
Amphibian     ancestry     of     mammals, 

theory  of,  333 
Amphignathodon,  199 
Amphioxus,  31,  32,  33,  34  *,  35,  36  *, 

37*,  38,  39*,  40*,  41*,  42,  43*, 

44  *,  45  *,  46  *,  47  *,  48,  71,  72,  75 
Amphioxus  theory  of  vertebrate  origin, 

72-75 

Amphiuma  means,  186,  187  * 
Amphiumidse,  186,  187 
Anabas  scandens,  108  *,  109,  150  * 
Anableps,  166 
Anacanthini,  111,  145,  146 
Anallanloida,  68,  69 
Anamnia  (Anamniota),  68,  69 
Anapsida,  215 
Anastomus,  beak  of,  282  * 
Ancistrodon  piscivorus,  257 
Angel-shark,  122  *,  123 


417 


418 


INDEX 


Anglers,  111,  150,  151 

Anguisfragilis,  250  *,  251 

Annelid   theory  of  vertebrate  origin, 

77  *,  78,  79 
Anseres,  296,  297 
Anseriformes,  289,  296,  297 
Ant-bears,  374,  375  * 
Ant-eaters,  adaptations  of,  19;  toads, 

202;    marsupial,    354,    355*,    356; 

Edentate,   374,   375*;  scaly,   375*, 

377 

Antelopes,  392 
Antiarchi,  82,  112,  171,  172 
Anthropoidea,  346,  378,  380-388 
Anura,  196-203 
Apes,    378;    New    World,    381;    Old 

World,    381-384;    anthropoid,    381- 

384 

Ape-man,  387 
Apoda,  184-186 
Apodes,  111,  140,  141 
Appendicularia,  59  *  (see  Larvacea) 
Apterygiformes,  286,  287 
Apteryx  australis,  285  *,  286,  287 
Arachnid  theory  of  vertebrate  origin, 

79-82 

Arachnid  and  vertebrate  compared,  81  * 
Arboreal  origin  of  flight,   theories  of, 

273-275 
Archceopteryx,  272,  273,  274  *,  275,  276- 

278;  A.  lithographica,  276,  277  *,  278  * 
Archseornithes,  276-278 
Archaic,  birds,  289;  mammals,  339-344 
Archegosaurus,  181 
Archenteron,  47 
Arctomys  candatus,  372  * 
Arius,  eggs  of,  164 
Armadillos,  376 
Aromochelys  odorata,  237 
Arthrodira,  112,  172 
Arthropod    ancestry    of    vertebrates, 

theory  of,  79-82 
Artiodactyla,  346,  389-392 
Ascidia  mammillata,  52  * 
Ascidiacea,  33,  50  *,  51  *,  52  *,  53,  54  *, 

55  *,  56  * 

Ascidise  compositae,  55  * 
Ascidiae  lucise,  55,  56  * 
Ascidian,  a  typical,  50  *,  51  *;  devel- 
opment of,  52  *,  53,  54  *;  colonial, 

55  *,  56  *;  tadpole  larva  of,  52  *,  53 
Aspidonectes  spinifer,  240  *,  241 


Asymmetron,  35,  48 

Asymmetry,  of  Amphioxus,  48 

Ateles  ater,  379  * 

Athecse,  234-235 

Atrial  funnel  (see  Atriopore) 

Atrial  cavity,  39,  51 

Atriopore,  39,  50,  51  * 

Atrium,  of  Amphioxus,  38,  39,  40  * 

Auditory  capsules,  in  Squalus,  114 

Auditory    organs,    of    fishes,    101;    of 

Squalus,  118 
Auk,  Great,  302 
Auricularia  larva,  64 
Autostylic  (skull),  125 
Aves,  33,  69,  260-323;  characterized, 

260;  compared  to  aeroplane,  260-265; 

compared  with   reptiles,    265,    266; 

list  of  characters  of,  266-271;  origin 

of,  271-276 

Axial  gradient  theory,  9-11 
Axial  gradient  in  development,  9,  10 
Axis,  of  polarity  (primary  axis),   1-3; 

antero-posterior,     1-3;     apico-basal, 

1-3;  dorso-ventral  (secondary  axis), 

4  *;  bilateral  (tertiary  axis),  4,  5  * 
Axolotl,  189,  190  * 
Aye-aye,  380 

Baboons,  381 

Backward  retreat  of  lower  functions,  8 

Badger,  367  *,  368 

Balcena  australis,  401  *;  B.  mysticetus, 

403* 

Balceniceps,  beak  of,  282  * 
Balanoglossus,  31,  60,  61  *,  62  *,  63,  65, 

83;  B.  clavigerus,  62  * 
Baleen,  403 

Ballistapus  rectangulus,  151  * 
Bandicoots,  355  *,  356 
Baptanodon,  218  * 
Baptornis,  279,  281 
Bartlett,  196,  198 
Basiliscus  americanus,  248  *,  252 
Basilisk,  helmeted,  248  *,  252 
Bass,  111,  146 
Bats,  363-365 
Bat-Fish,  150 

Bathymal  Sea  Devils.  Ill,  151 
Batoidei,  111,  123-125 
Bdellostoma,  91  *;  eggs  of,  164  * 
Bdellostomidse,  106 
Beebe,  C.  W.,  273 


INDEX 


419 


Belone,  111,  145 

Bendire,  Major,  298 

Bernicla  ruficollis,  293  * 

Birds  (see  Aves);  archaic,  276,  277; 
modern,  278-323;  feet  of,  280  *; 
beaks  of,  282  *;  present  status  of,  281 ; 
sex-dimorphism  of,  281;  of  Paradise, 
310,  311  *;  future  of,  312,  313; 
migration  of,  313;  geographic  dis- 
tribution of,  314 

Birkenia,  169  *,  171 

Bitterns,  295 

Blastodermic  vesicle,  of  rabbit,  408  * 

Blastopore,  47 

Blastula,  of  Amphioxas,  42,  43  *;  of 
frog,  205,  206  * 

Blennius,  eggs  of,  164 

Blind-worms,  184 

Boa-constrictors,  258 

Boidce,  258 

Bombinator  igneus,  197  *,  198 

Bothriolepis,   80*,   81,   82,    169*,    171 

Botryllus  violaceus,  55  * 

Boulenger,  137 

Bovidae,  390,  392 

Bow-fin,  111,  134  * 

Brachiosaurus,  218,  221 

Bradipodidse,  374 

Bradypus,  376 

Brain,  Amphioxus,  38,  39  *;  of  Cylos- 
tomata,  88;  of  Myxinoidea,  90;  of 
Lamprey,  92,  93*;  of  Pisces,  101; 
of  Elasmobranchii,  118  *;  of  Scyllium, 
118  *;  of  Teleostomi,  128;  of  turtle, 
231;  of  alligator,  244  *;  of  pigeon,  270, 
271;  of  rabbit;  330  *;  of  dog,  332  *; 
of  mammals,  331;  of  Ornithorhyn- 
chus,  351  * 

Branchial  basket,  of  Cyclostomata,  88; 
of  myxinoids,  90 

Branchial  clefts,  of  cyclostomes,  88 

Branchiostoma  (see  Amphioxus) 

Branchiosaurus,  180,  181  *;  B.  ambly- 
stomas,  182*;  B.  salamandroides,  182* 

Breeding  habits,  of  Amphioxus,  42; 
of  lamprey,  94  *;  of  Amia,  134;  of 
stickle-back,  144;  of  sea-horse,  144; 
of  Rhodeus,  135  *,  137;  of  salmon, 
138,  139;  of  killifishes,  142,  156;  of 
cod,  146;  of  Lepidosiren,  158,  159; 
of  bass  and  pickerel,  166;  of  Ichlhyo- 
phi$,  185  *,  186;  of  Desmognathus, 


188  *;  of  Pipa  americana,  196,  197  *, 
198;  of  Alytes,  197*,  198,  199;  of 
Hyla  faber,  201;  of  Sceloporus,  251; 
of  Echidna,  350;  of  Ornithorhynchus, 
351,  352 

Bridge,  T.  W.,  156,  157,  158 

Brontosaurus  *,  220 

Brood-pouch,  of  sea-horse,  144  * 

Bubo  virginianus,  307  * 

Bufo  lentiginosuSj  197  *,  199;  B. 
vulgaris,  199 

Bufonidse,  199 

Burbot,  146 

Bustard,  great,  301 

Bustard  quails,  299        ^         — -\ 

Buzzard,  Turkey,  297,  298  /- 

Cachalot,  400,  401  *,  402 

Cacops,  180  *,  183 

Ccecilia,  185  * 

Caecilians  (see  Apoda) 

Caenogenetic  characters,  24 

Ccenolestes,  357 

Calamichthys,  111,  128,  129,  130  *,  159 

Callechelyx  luteus,  159 

Callorhynchus  antarclicus,  126  * 

Camel-birds  (see  Ostriches) 

Camels,  389,  390;  Bactrian,  391  * 

Camelus  bactrianus,  391  * 

Canidse,  366,  367  *,  368 

Canis  nubilis,  367  * 

Capitulum,  326 

Capra  sinaitica,  391 

Caprimulgi,  306 

Capuchins,  378,  381 

Caracara,  298 

Carapace,  of  turtle,  226,  227  *,  228 

Carinatse    (see  Neornithes  Carinatae); 

classification  of,  289 
Carnivora,  364-371;  fissiped  carnivores 

364-368;  pinniped  carnivores,  388- 

371;  extinct,  370 
Carp,  111,  140 
Cassowary,  284,  285  *,  286 
Cast  or  fiber,  372  * 
Casuarias  uniappendiculatus,  285  * 
Catarrhini,  378,  381-384 
Cat-fishes,  111,  140 
Catosteomi,  111,  143,  144,  145 
Cats,  366 
Caudal  fin,  of  fishes,  100;  types  of,  104, 

105  *;  of  Holocephali,  127 


420 


INDEX 


Cave  and  deep-sea  animals,  18 

Caviar,  133 

Cavies,  373 

Cebidse,  378,  381 

Centurio  senex,  365  * 

Cephalaspis,  169  * 

Cephalization  in  Vertebrates,  7,  8 

Cephalochordata,  33 

Cephalodiscus,  31,  32,  33,  60,  65  *,  66  * 

Ceratobranchinae,  202 

Ceratobranchus  guentheri,  202 

Ceratodus  (see  Neoceratodus) 

Ceratopsia,  223 

Cercopithecidae,  378,  381 

Cervulus,  390 

Cervus  canadensis,  391  * 

Cetacea,  346,  400-404 

Chcelopus  didactylus,  375  *,  377 

Chceropus  castanotis,  356 

Ckamceleon    vulgaris,    253,    254*,    C. 

pumulis,  254  * 
Chamceleontes,  253-255 
Chamberlin,    T.    C.,    theory    of    the 

stream  origin  of  fishes,  73-75 
Chameleons,  253-255 
Characinidse,  140 
Charadriiformes,  289,  301-305 
Chel-odina,  241 
Chelone  imbricata,  235  *,  240;  C.  mydas, 

240 

Chelonia,  216,  233-242 
Chelonidse,  239,  240 
Chelydosaurus,  181 
Chelydra  serpentina,  235  *,  236,  237 
Chelydridse,  236-237 
Chelys  fimbriata,  240  * 
Chevrotani,  389 
Child,  C.  M.,  vii,  8,  10,  161 
Chimseras,  111,  125-127 
Chimceramonstrosa,  126  *;  C.  colliei,  127 
Chimpanzee,  378  *,  384 
Chinchilla,  373 
Chirogale  coquereli,  380 
Chiromys  madagascariensis,  380 
Chironectes,  354;  C.  minima,  355  * 
Chiroptera,  346,  363-365 
Chlamydophorus,  376 
Chlameidoselachus,    107,    118;    C.    an- 

guinius,  122  * 
Chondrostei,  104,  106,  111 
Chordata,  the  phylum  characterized, 

31;  classification  of,  33 


Chorion,  in  birds,  319,  320  *;  in  mam- 
mals, 408,  409 

Chrysemys  picta,  238;  C.  marginata,  238 
Chrysochloris  trevelyani,  361  *,  362 
Ciconia  alba,  293  * 
Ciconiiformes,  289,  295,  296 
Cinosternidae,  237 
Cinosternum  pennsylvanicum,  235  * 
Circulatory  system,  of  Amphioxus,  39, 

41*;  of  Pisces,  100;  of  Squalus,  116; 

of  Amphibia,  177;  of  turtle,  231;  of 

crocodile,  243;  ^of  bird,  267,  269  *, 

270;  of  mammal,  333 
Cistudo  Carolina,  235  *,  238,  239 
Civets,  366 
Cladista,  111,  129 
Cladoselache,  71,   103,   105;  C.  fyleri, 

119  *,  120 

Cladoselachidae,  110,  119 
Clarias,  108,*  109 
Claspers    of    sharks,    113;    of    Holo- 

cephali,  125,  126 
Cleavage,  of  Amia,  165;  of  Neoceratodus, 

166  *,  167,  168;  of  Lepidosteus,  165; 

of  frog,  204,  205,  206*;  of  chick, 

316  *,    317;   of   placental  mammal, 

407  *;  of  rabbit,  407  * 
Climbing  Perch,  150  * 
Clupeidse,  138,  139 
Cobra,  256  *,  259 
Coccosteus,  171  *,  172 
Cod,  145,  146  * 
Coalom,  1,  42;  development   of,  45  *, 

46* 

Coelomoducts,  41 
Colii,  308 
Coloration,  of  fishes,  110;  of  flounders, 

148;  of  birds,  312 

Columba,  beak  of,  282  *;  C.  livia,  304 
Columbse,  302,  304 
Colubrinae,  258 
Colymbiformes,  289,  292,  293 
Colymbimorphae,  289 
Colymbus  gracilis,  293  * 
Condylarthra,  340,  342  * 
Coneys,  398,  399  * 
Continuous   fin-fold    theory,    102;    of 

Wiedersheim,    103  *;    of    Kingsley, 

104* 

Convergence  or  parallelism,  law  of,  16 
Cope,  E.  D.,  137 
Copperhead,  257 


INDEX 


42J 


Copulation,  in  elasmobranchs,  113, 114; 
in  Preciliidae,  142, 166;  in  snakes,  256 

Coracise,  306  * 

Coracias  garrulus,  307  * 

Coraciiformes,  289,  306-309 

Coraciomorphse,  289 

Cormorants,  295 

Corpus  callosum,  325,  330  *,  331 

Coryphodon,  342,  343  * 

Corythosaurus,  218,  221 

Costa,  33 

Coues,  292,  293 

Coyote,  366 

Cranes,  sandhill,  301 

Craniata,  classification  of,  33 

Cranium,  of  Cyclostomata,  88;  of 
Lamprey,  93;  of  Polypterus,  130*, 
131;  of  stegocephalian,  182*;  of 
turtle,  229,  230*;  of  Sphenodon, 
233  *;  of  Archceopleryx,  278;  of 
mammal,  328  *;  of  cynodont,  334  *; 
of  Ptilodus,  338;  of  Balcena,  403 

Creodonta,  340,  341  *,  342 

Crex  pratensis,  303  * 

Cricotus,  180  *,  183 

Crocodilus  americanus,  245;  C.  niloticus, 
246 

Crocodilia,    242-246;    habits    of,    244 

Crocodilidae,  245,  246 

Crocuta  maculata,  367  * 

Crossopterygii,  105,  106,  111,  127,  128- 
131;  as  ancestors  of  Amphibia,  174, 
175 

Crotalus  durissus,  256  *,  258 

Cryptobranchus   allegheniensis,    186, 
187  *;  C.  japonicus,  186,  187 

Cryptodira,  236-240 

Cryptopsarus  couesii,  136  * 

Crypturiformes,  287,  288 

Ctenoid  scales,  109,  110 

Cuckoos,  305 

Cuculi,  305 

Cuculiformes,  289,  306,  307 

Cursorial  origin  of  flight,  theory  of, 
272-273 

Cycloid  scales,  109, 110 

Cyclomyaria,  60 

Cyclopes^  37± 

Cyclostomata,  33,v/68,v  69,  73,  87-96 

Cynpdontia,  216,  333,   334,  335,  336 

Cynognathus,  215  *,  216 

Cyprmidse,  140 


Cyprinodon,  166 

Cypselus,  foot  of,  280  *;  beak  of,  282  * 

Cystignathidse,  201,  202 

Dasypodidse,  376 

Dasyprocta  aguti,  372  * 

Dasypus  novemcinclus,  375  *,  376 

Dasyuridae,  356 

Dasyurus  viverrinus,  355  *,  356 

Deal-Fish,  150 

Deep-sea  Fishes,  136  * 

Deer,  Mouse,  389;  family,  390;  musk, 

390 
Degeneracy,     a     criterion     of     racial 

senescence,  21 
Delphinus  delphis,  401  * 
Dendrobatinae,  202,  203 
Dendrobatinus,  203 
Dendrochirus,  104;  D.  hudsoni,  149  * 
Dendrolagus,  359 
Dentition,  of  mammal,  328-331 
Dermatemydae,  237 
Dermochelys  coriacea,  234,  235  * 
Dermoptera,  346,  362 
Desert-dwellers  (adaptations  of),  18 
Desmodactyli,  309,  310 
Desmodus    rotundus,    364;    D.    rufus, 

365* 

Desmognaihus  fuscus,  188  * 
Development,  of  Amphioxus,  43-48;  of 

Tunicate,  52-54;  of  salpian,  57  *,  58; 

of  myxinoids,   91;  of  lamprey,   94, 

95  *,  96;  of  Neoceratodusforsteri,  166; 

of  salmon,  167;  of  Hylodes,  202  *;  of 

frog,  203-209;  of  birds,  314-323;  of 

mammals,  404-411 
Devil  fish,  125 
Diadectes,  216 

Diaphragm,  of  mammals,  325 
Diapsida,  214 
Diemictylus  viridescens,  191 
Didelphia,  345,  352-359;  definition,  352 
Didelphidse,  354-355 
Didelphis  virginiana,  354,  355  * 
Dimetrodon,  215  *,  216 
Dingo  dog,  366 
Dinoceras,  343  * 
Dinornis,  287 
Dinornithiformes,  287 
Dinosaurs,     Carnivorous,     219,     220; 

Herbivorous,  220-223;  extinction  of, 

223,  224 


422 


INDEX 


Diomedea  exultans,  293  * 

Diphy cereal  caudal  fin,  104,  105  *;  in 
Pleuracanthus,  121 

Diplacanthidae,  110,  119 

Diplocaulus,  180  *,  181 

Diplosoma,  55  * 

Dipneusti,  107,  112,  154-159 

Dipnoi,  112 

Diprotodontia,  345,  357,  358  *,  359 

Dipterus,  156 

Dipus  jaculus,  372  * 

Discoglossidse,  198,  199 

Dissorophus,  181 

Distcechurus,  357 

Divergence,  law  of  (adaptive  radia- 
tion), 19 

Diving  origin  of  flight,  theory  of,  275, 
276 

Dodo,  304 

Dog-fishes,  113 

Dog-sharks,  112 

Dolichoglossus  kowalevskii,  62  * 

Doliolum,  56,  57  *,  58;  life  cycle  of, 
57  *,  58;  D.  tritonis,  57  * 

Dollo,  235 

Dominance  and  subordination,  11 

Draco  volans,  248  *,  249,  250 

Drepanaspis,  169  *,  170 

Dromceus  novce-hollandioe,  285  * 

Dromicia,  357 

Dromocyon,  341  *,  342 

Dromotherium  sylvestre,  jaw  of,  337  * 

Ducks,  297 

Dugongs,  396,  397,  398  * 

Duplicidentata,  371 

Eagle,  golden,  298 

Eagle  ray,  124  *,  125 

Echidna  aculeata,  348,  349  * 

Echidnidsa,  345,  348-350 

Edaphosaurus,  216 

Edentata,  345,  373-377;  extinct,  377 

Eels,  111,  140,  141;  electric,  140 

Eel-like  form,  a  criterion  of  racial 
senescence,  21 

Eggs,  of  Amphioxus,  42;  of  tunicate,  52; 
of  myxinoids,  91;  of  lamprey,  94;  of 
fishes,  101;  of  Teleostomi,  128;  of 
sturgeons,  133;  of  Amia,  134;  of 
fishes,  163,  164  *,  165;  of  ling,  165; 
of  Lepidosteus,  165;  of  frog,  204;  of 
Chelydra,  236;  of  Aromochelys,  238; 


of  Aspidonectes,  242;  of  Ostrich,  283; 

of  bird,  315  * 
Elaps,  257 

Elasmobranchii,  112-127 
Elasmosaurus,  218  *,  219 
Electric  organs,  of  rays,  123 
Electric  rays,  123,  124  * 
Elephant  birds,  287 
Elephantidse,  394,  395  *,  396;  extinct, 

396 
Elephas  indicus,  394,   395*;  E.  afri- 

canus,  394,  395  * 
Eleutherodactyli,  310 
Elk,  390,  391  * 
ElopidaB,  138 
Emberiza,  beak  of,  282  * 
Embryo,  of  chick  (35  Somites),  318  *; 

five  days  old,  321  *;  of  Hypsiprym- 

nus,  405*;  of  Phascolarctus,  405*; 

of  Perameles,  406  * 
Embryonic  membranes,  of  reptile,  212, 

213*;  of  bird,   319,  320*,   321;  of 

marsupials,    405  *,    406;    of   rabbit, 

410  *;  of  man,  411  * 
Emeu,  284,  285  *,  286 
Emys  orbicularis,  235  * 
Endocrine    glands    and    racial    senes- 
cence, 22 
Endostyle,    37,    38,   40*,    51,    96;   of 

Ammocoetes,  95,  96 
Engystoma  carolinense,  202 
Engystomatidse,  202 
Enteropneusta,  33,  84,  85 
Environment  in  relation  to  evolution  of 

characters,  28,  30 
Epanorthida^  357 
Eptesicus  fuscus,  364 
Equidse,  392 

Equus,  392;  E.  burchelli,  393  * 
Erinaceidae,  362 
Erinaceus,  362 
Eryops,  180  *,  183 
Esocidae,  142 
Esox  masquinongy,  142 
Eudyptes  chrysocoma,  293  * 
Eulampis  jugularis,  307  * 
Eumetopias  jubata,  369  * 
Eunotosauria,  234 
Euphractus  sexcinctus,  376 
Eusuchia,  242 

Eutheria,  345,  352;  definition,  352 
Evans,  A.  H.,  289 


INDEX 


423 


Evolution  of  foot  from  fin,  176  * 
Evolutionary  advances  of  reptiles,  211- 

212 

Exonautes  gilberti,  145  * 
External  gills,  106  *,  107, 108, 185  *,  207 
Eyes,  of  Amphioxus,  38;  of  myxinoids, 

90;  of  lampreys,  92;  of  fishes,  101; 

of  bird,  271 

Falcinellus,  beak  of,  282  * 

Falco,  foot  of,  280  *;  beak  of,  282  * 

Falconiformes,  289,  297,  298 

Falcons,  298 

Faserstrang,  77 

Fierasfer,  111,  142;  F.  acus,  142  * 

.Feathers,  of  pigeon,  261;  266  *,  267 

Feet,  of  Birds  280  *;  of  mammals,  337 

Felidse,  366 

Felis  canadensis,  367  *;  F.  caffra,  366 

File-fishes,  112 

Finfoot,  301 

Fins,  of  fishes,  100,  102-105 

Fishes,  coloration  of,  110;  integument 
of,  109,  110;  life  zones  of,  97;  types 
of  body  form,  97,  98,  99  *;  structural 
features  of,  100-102;  fins  of,  102-105; 
respiratory  organs  of,  106-109;  swim- 
bladder  of,  108,  109;  classification  of, 
110-112 

Fissipedia,  364-368 

Flamingoes,  295,  296 

Flat-fishes,  147 

Flounders,  111,  146,  147  * 

Flower  and  Lydekker,  376 

Flying  dragon,  248  *,  249,  250 

Flying-fishes,  145  *,  150 

Flying  frog,  203 

Flying  gurnards,  150 

Food  concentrating  mechanism,  of 
Amphioxus,  36,  37;  of  tunicates,  50, 
51* 

Fool-fishes,  151 

Fossa,  366 

Four-wing  theory  of  origin  of  flight, 
273-275 

Frigate  birds,  295 

Frilled  shark,  122  * 

Frog,  bull,  200  *,  203;  leopard,  200;  * 
Javan  flying,  200  *,  203;  European 
brown,  203;  cricket,  201 

Fuertes,  L.  A.,  figures  after,  367*, 
369  *,  391,  398 


Fulica,  foot  of,  280  * 
Fundulus  heteroclitus,  142*,  160,  161, 
166 

Gadow,  H.,  183, 184,  225,  226,  232,  253> 
254,  256 

Gadus  morrhua,  146  * 

Galeopithecus  volans,  362  *,  363 

Gall  bladder,  of  Ammoccetes,  95,  96;  of 
Squalus,  116 

Galli,  299,  300 

Galliformes,  289,  299,  300 

Gallus  gallus,  300 

Gambusia,  166 

Gannets,  295 

Ganoid  scales,  109,  110 

Ganoin,  110 

Gar-pike,  111,  133 

Garrod  and  Forbes,  309 

Gaskell,  80 

Gasterosteus  aculeatus,  144  * 

Gastrostomus  bairdii,  136  * 

Gastrula,  of  Amphioxus,  42,  43  *;  of 
teleost,  167*,  168;  of  frog,  205, 
206  *;  in  the  bird,  317 

Gavialidse,  244-245 

Gavialis  gangeticus,  244,  245  * 

Geckones,  247 

Geckos,  247;  wall  gecko,  248  * 

Geese,  297,  295  * 

Generalized  types  of  vertebrates,  12, 
13;  of  fishes,  159-160;  of  birds,  312 

Geologic  time,  main  divisions  of  (chart), 
29*;  scale,  26* 

Gephyrocercal  caudal  fin,  131 

Giant  land  tortoises,  239,  240  * 

Giant  size,  a  criterion  of  racial  senes- 
cence, 20,  21 

Giantism,  possible  causes  of,  22 

Gibbon,  378  *,  382 

Gila  monster,  250*,  252 

Gill,  T.  N.,  141 

Giraffa  camelopardus,  391  * 

Giraffe,  390,  391  * 

Glandiceps  hacksi,  62  * 

Glass  snake,  250*,  252 

Globe-fishes,  152 

Glossobalanus,  64,  84 

Glyptodon,  377 

Gnathonemus,  139 

Gnathostomata,  69 

Goats,  392 


424 


INDEX 


Goat-suckers,  306 

Gobiidae,  149 

Goby,  111,  146,  149 

Goethe,  7 

Golden  Age  of  Reptiles,  217 

Gonads,  of  Amphioxus,  42;  of  Cyclos- 

tomata,  88;  of  fishes,  102 
Goodrich,  154 
Gorilla  gorilla,  384,  385  * 
Graptemys,  227  *;  G.  geographica,  238 
Grebes,  292,  294 
Gregory,  W.  K.,  theory  of  origin  of 

flight,  275;  378,  387 
Gruiformes,  289,  301 
Guinea-fowls,  299 
Gulls,  302 
Gulper-eels,  141 
Gymnarchus,  139 
Gymnophiona  (see  Apoda) 
Gymnothrax  waiahuz,  141  * 
Gymnotidae,  140 
Gymnotus  elect?  icus,  140 
Gymnura,     360,     361  *;    G.    rafflesii, 

361  *,  362 
Gypogeranus,  298 
Gyrfalcons,  298 

Haddock,  146 

Hag-fishes,  87,  89  *,  90,  91  * 

Hake,  146 

Halicore,  398,  399  * 

Hammer-head  sharks,  121,  122  * 

Hapalida,  378,  381 

Haplomi,  111,  142 

Harrimania,  84 

Harriota  raleighana,  126  * 

Harrington,  129 

Hatteria  (see  Sphenodon) 

Hawksbill  turtle  (see  Chelone) 

Head-fishes,  98,  152,  153  *,  161 

Heart,  of  crocodile,  243  * 

Hedgehogs,  360,  362 

Hellbender  (see  Cryptobranchus) 

Heloderma  horridum,  250  *,  252 

Hemichordata,  33,  60,  61  *,  62  *,  63  *, 

64,  65  *,  66  *,  67  *,  68 
Hemimyaria,  60 
Hemipodes,  299 
Heptanchus,  106 
Hermaphrodism,  in  tunicates,  52;  in 

hag-fishes,  90 
Herons,  295 


Herrings,  111,  139 
Hesperornis,  279  *,  280,  281 
Heterocercal  caudal  fin,  104,  105  * 
Heterodontus,  eggs  of,  164 
Heteromi,  111,  142,  143 
Heterosomata,  148 
Heterostraci,  168,  169,  170 
Hexanchus,  106 
Hippocampus  guttatus,  144  * 
Hippopotamus    amphibius,  391  * 
Hirudo  rustica,  311  * 
Hoactzin,  300 
Hogs,  389;  European  wild,  389;  wart 

389,  391  * 
Holocephali,  104,  106,  107,  111,  125- 

127 

Holostei,  104,  106,  133,  134 
Hominidse,  378,  385-388 
Homo  sapiens,  386;  varieties  of,  386, 

387;  H.  neanderthalensis,  387,  388  * 
Homocercal  caudal  fin,  105  * . 
Homologies  as  evidence  of  phylogeny, 

23 

Hornbill,  303  *,  306 
Horses,  392 
Hubrecht,  82 

Humming  birds,  307  *,  308 
Hutton,  294,  295 
Huxley,  T.  H.,  333 
Hyamas,  366,  367  * 
Hyaenidae,  366 
Hycenodon,  341  * 

Hydromedusa  maximiliani,  240  *,  241 
Hyla  versicola,    199,   200*;   H.  faber, 

201;  H.  gcsldii,  201*;  H.  arborea, 

200* 

Hylidae,  199-201 
Hylobates  lar,  379  *,  382 
Hylodcs  martinicensis,  202  * 
Hylomys,  362 
Hypochordal  rays,  104 
Hypsiprymnus,  embryo  of,  405  * 
Hyracoidea,  346,  398,  399 
Hyrax  abyssinicus,  399  * 
Hystricomorpha,  373 
Hystrix  cristata,  372  * 

Ibex,  391  * 

Ibis,  295 

1  chthyophis  glutinosus,  185  *,  186 
Ichthyopsida,  68,  69 
Ichthyorniformes,  289-291 


INDEX 


425 


Ichthyornis  victor,  290  * 

Ichthyosauria,  17  *,  216,  217,  218 

Ichthyotomi,  110 

Icterus  baltimore,  311  * 

Idiacanthus  ferox,  136* 

Iguana  tuberculata,  248  *,  253 

Iguanodon,  221,  222  * 

Incus,  325 

Insectivora,  340,  346,  360 

Integument,  of  fishes,  109,  110;  of 
Teleostomi,  128;  of  turtle,  226-228; 
of  Crocodilia,  242,  243;  of  mam- 
mals, 325-328;  of  armadillos,  376 

Internal  gills,  107  *,  108;  of  frog  larva, 
207,  208  * 

Isospondyli,  137 

Jacana,  302 

Jerboa,  372  * 

Jordan,  D.  S.,  137,  138,  144,  147 

Jungle-fowls,  299,  300 

Kangaroos,  358  *,  359 

Keeled    Birds    (see    Neornithes    car- 

inatae) 
Kellicott,  W.  E.,  viii,  43,  44,  45,  46, 

47 

Kestrels,  298 
Killifishes,  111,  142  * 
Kingsley,  J.  S.,  viii,  103 
Kite,  red,  293  * 
Kiwi  (see  Apteryx) 
Knowlton,  F.  H.,  289,  305 
Koala,  358  *,  359 

LaUdosaurus,  215  *,  216 

Labyrinthodonta,  181 

Lacerta  viridis,  248  *,  249 

Lacertse,  247-253 

Lacertilia,  246-255 

Lagomyidse,  371 

Lampreys  (see  Petromyzontia) 

Lanarkia,  169  *,  170 

Lancelets  (see  Amphioxus) 

Lappet-fins,  of  Cladoselache,  120 

Lari,  302 

Larva,  of  Amphioxus,  46  *,  47  *;  of 
Petromyzon,  38;  of  Polypterus,  129, 
130  *;  of  Protopterus,  155  *,  156;  of 
Lepidosiren,  155  *,  156;  of  frog, 
206  *,  207,  208 

Larvacea,  33,  50 


Lasanius,  169  *,  171 

Lateral  line  organs;  of  Cyclostomata, 

88;  of  fishes,   101;  of  Squalus,   118 
Leatherback  turtle,  234-235  * 
Lemur  varius,  380 
Lemur,  378,  379  *,  380;  Smith's  dwarf, 

379  *;  ruffed,  380;  mouse,  380 
Lemuroidea,  346,  378,  379  *,  380 
Lepidosiren  paradoxica,  112,  155  *,  156, 

158,  159 

Lepidosteidse,  133 
Lepidosteus,  134  *;  eggs  of,  164 
Leporidse,  371 
Life  zones  of  fishes,  97 
Lillie,  F.  R.,  24 

Limax  lanceolatus  (see  Amphioxus) 
Limicolae,  302 
Limulus,  77,  79,  170 
Linophryne  lucifer,  136  * 
Lissamphibia,  180,  183 
Liver  diverticulum  of  Amphioxus,  34  *, 

39 
Liver,  of  Ammocoetes,  95;  of  Squalus, 

116 
Lizards,    246-255;    wall,    248*,    249; 

Galapagos  sea,  250  *,  252 
Lizard-tailed  bird,  277,  278 
Llamas,  390 
Lobe-fin,  105 
Lobe-finned  ganoids   (see  Crossopter- 

ygii) 

Loons,  292,  293  *,  294 
Loricata,  376,  377 
Loxomma,  180,  182 
Lull,  R.  S.,  viii,  20,  21,  79-81,  223,  224, 

338,  339 

Lung-fishes,  112,  154-159 
Lutra  canadensis,  367  * 
Lyre-birds,  310,  311  *,  312 

Macaques,  381 

Machaerodontia,  370 

Mackerel,  111,  147 

Macrochelys  temmincki,  236 

Macropodidse,  359' 

Macropus  rufus,  358  *,  359 

Mceritherium,  396,  397  * 

Malacopterygii,  111,  135,  137-139 

Malleus,  325 

Mammalia,  13,  33,  68,  69,  324-411; 
characterization  of,  324,  325;  dis- 
tinguishing characters  of,  325,  326; 


426 


INDEX 


Mesozoic,  336-339;  Cenozoic,  339- 

404;    classification    of    modernized, 

345,  346;  placental  mammals,  360- 

404 

Mammary  glands,  325 
Mammoth,  396 
Manatees,  396,  397,  398  * 
Mandril,  379  *,  381 
Manidae,  377 
Manis,    377;    M.    gigantea,    377;    M. 

temmincki,  375  *,  377 
Marmosa,  354 
Marmosets,  377,  381 
Marmot,  372  * 
Marsipobranchii,  88 
Marsupialia,  345 
Mastodon,  396 
Matamata,  240  * 
Matthew,  W.  D.,  219,  220 
McGregor,  J.  H.,  387,  388 
Medullary    Plate,    definition    of,    32, 

45  *,  47 

Megachiroptera,  363 
Megalichthys,  182 
Megalops  atlanticus,  138  * 
Melurus,  368 
Menura  superba,  311  * 
Merganser,  297 

Mergius,  foot  of,  280  *;  beak  of,  282  * 
Merostomata,  79,  80  *,  81,  82 
Messenatides,  299 
Mesite,  Madagascar,  299 
Metamerism  in  Vertebrates,  6,  7 
Metamorphosis,  of  Amphioxus,  48;  of 

tunicate,  53,  54  *;  of  lamprey,  96;  of 

frog,  207,  208  *,  209 
Metatheria,  345,  352-259;  definition, 

352 

Miastor,  195 
Microcebus  smithii,  379  * 
Microchiroptera,  363-365 
Micromeres,  42 
Micropodii,  308 
Microstomum,  11 
Milvusictinius,  293  * 
Moa,  287 
Moles,  pouched,  356;  true,  360,  361  *, 

362 

Moloch  horridus,  250  *,  252 
Mollosidse,  364 
Mongoose,  366 
Monitor,  Cape,  250  *,  252,  253 


Monkeys,  howler,  378,  381 ;  spider,  378 
Monodelphia,     345,     360-404;    defini- 
tion, 360 

Monotremata,  345,  347-352 
Moose,  390 
Moray,  141 

Morula,  of  rabbit,  407  * 
Mucous  rope  of  Amphioxus,  37,  38 
Mucous  sacs,  of  myxinoids,  90 
Mud-minnow,  142 
Mud-puppy  (see  Necturus) 
Miiller,  Johannes,  64 
Multituberculata,  340 
Muskalunge,  142 
Mustelidae,  368 
Mustelus,  113 

Mycteria,  foot  of,  280  *;  beak  of,  282  * 
Myliobatida?,  124, 125 
Myliobatis  aquila,  124  * 
Mylodon,  377 
Myocommata,  42 
Myomorpha,  373 
Myotis  lucifugus,  364 
Myotomes,  42 
Myrmecobiidae,  354-356 
Myrmecobius  fasciatus,  354,  355  *,  356 
Myrmecophaga  j ubata,  374,  375  * 
Myrmecophagidae,  374 
Mystacoceti,  346 
Myxine,  89  *,  90;  eggs  of,  164  * 
Myxinidse,  106 
Myxinoidea,  89  *,  90,  91  *,  92 

Naja  tripudians,  256  *,  259 

Nannemys  guttata,  238 

Narwhal,  402,  403 

Nasal  apertures,  in  Squalus,  114 

Nasal  sacs,  of  Teleostomi,  128 

Naso-pituitary  sac,  90,  92 

Necturus  maculatus,  192,  193  * 

Nematichthys,  141 

Nemertean  theory  of  vertebrate  origin, 

82,  83  * 

Neoceratodus,  112,  155  *,  157,  166,  168 
Neornithes,    278-323;   N.    Odontolcae, 

279,  280,  281;  N.  Ratitse,  279,  281- 

288;  N.  Carinatse,  279,  288-312 
Neoteny  (see  Paedogenesis) 
Nephridia,    of    Amphioxus,    41  *;    of 

Squalus,  116;  of  fishes,  102 
Nests,  of  lamprey,  95;  of  stickleback, 

144;  of  Amia,  134;  of  Hylafaber,  201; 


INDEX 


427 


of  Chelydra,  236;  of  Aromochelys,  238; 
of  Chrysemys,  238;  of  Aspidonectes, 
242;  of  alligator,  245,  246;  of  croco- 
dile, 247;  of  lizard,  249;  of  flamingo, 
296;  of  hornbill,  306;  of  birds,  323; 
of  Ornithorhynchus,  352 

Neural  groove,  32,  45  * 

Neural  tube,  32,  45  * 

Neurenteric  canal,  47 

Neuroccel,  32,  45  * 

Nictitating  membrane,  of  turtle,  226 

Nidicolse,  323 

Nidifugae,  323 

Nopcsa,  F.,  272 

Northern  diver,  293  * 

Nostril,  median,  of  myxinoid,  90 

Notochord,  definition  of,  31;  of  Am- 
phioxus,  38,  45  *;  development  of, 
45  *;  of  Cyclostomata,  88 

Notodanidae,  112 

Notoryctes  typhlops,  355  *,  356 

NotoryctidoR,  356 

Nototrema  marsupium,  200  *,  201 

NuttaJl,  304 

Nycticebus  tardigradus,  380 

Nythosaurus  larvatus,  334  * 

Odontocetse,  346,  400,  402 

Odontplcse     (see     Neornithes    Odon- 

tolcse) 

Ogilvie-Grant,  299 
Oikopleura,  59  * 

Olfactory,  funnel  of  Amphioxus,  38 
Olfactory  lobes,  of  Squalus,  118 
Olfactory  nerves,  of  Amphioxus,  38 
Olfactory   organs,    of   fishes,    101;    of 

Squalus,  118 
Olms  (see  Proteus} 
Operculum,  107;  of  Holocephali,  126; 

of  Teleostomi,    127;  of  frog  larva, 

207 

Ophidia,  255-259 
Opisthocomi,  300 
Opisthoglyphs,  257 
Opisthomi,  111,  150 
Oral,    funnel    of    Amphioxus,    38;    of 

tunicate,  50,  51  *;  of  lamprey,  92 
Oral  hood  (see  oral  funnel),  48 
Orang-utang,  382,  383  * 
Orca  gladiator,  401  * 
Orientation  of  embryo  in  bird's  egg, 

317* 


Origin,  of  fishes,  72-75;  of  Amphibia, 
174,  175;  of  reptiles,  213,  214;  of 
birds,  271-276;  of  flight,  272-276;  of 
mammals,  332-336;  of  modernized 
mammals,  344;  of  Man,  387 

Oriole,  Baltimore,  311* 

Ornithischia,  219,  221 

Ornithopoda,  221 

Ornithorhynchidse,  345,  350-352 

Ornithorhynchus  anatinus,  157;  pectoral 
girdle  of,  347  *;  349  *,  350,  351,  352 

Orycteropus  capensis,  375  *,  377 

Osborn,  H.  F.,  viii,  16,  17,  19,  20,  25, 
26,  27,  29,  71,  86,  97,  98,  99,  176, 
210,  211,  212,  274,  340,  341,  345,  346 

Ostariophysi,  111,  140 

Osteolepida,  111,  129 

Osteolepis,  156 

Osteostraci,  170,  171 

Ostrachion  schlemmeri,  152  * 

Ostrachodermi,  80  *,  82,  112,  168-171 

Ostrich  Dinosaur  (see  Struthiomimus) 

Ostriches,  283,  284 

Otariidse,  368,  369 

Otter,  367  *,  368 

Owen,  R.,  7 

Owls,  306;  great  horned,  307  *;  screech, 
306 

Oxen,  392 

Paddle-fish,  111 

Paedogenesis,  14,  22;  in  Larvacea,  59; 
in  Ammoccetes,  96;  in  fishes,  163;  in 
urodeles,  189,  194,  195 

Pair  wing  theory  of  origin  of  flight, 

Palseogeography  in  relation  to  evolu- 
tion, 28 

Palceohatteria,  232 

PalcBomastodon,  396,  397  * 

Palaeospondylidse,  112 

Palingenetic  character,  24 

Pallas,  33 

Pan  pygmceus,  379  *,  384 

Pangolins,  377 

Papio  leucophceus,  379 

Paradisea  minor,  311  * 

Parapsida,  215 

Parasuchia,  216 

Parent  stream  theory,  138     • 

Parrots,  303  *,  305,  306 

Partridges,  299 

Passer  domesticus,  31 1  * 


428 


INDEX 


Passeriformes,  289,  309-312 

Passerine  birds,  309-312 

Patriofelis,  341 

Patten,  W.,  80-82,  171 

Pea-fowls,  299 

Pecora,  388,  391 

Pecten,  of  bird,  271 

Pectoral  fins,  100;  of  Squalus,  113 

Pectoral  girdle,  of  Squalus,  114;  of 
Teleostomi,  127;  of  turtle,  228,  229  *; 
of  Ornithorhynchus,  347  * 

Pediculati,  111,  150,  151 

Pelargomorphae,  289 

Pelicanus,  beak  of,  282  *;  295 

Pelobates  cultripes,  197  *,  199 

Pelobatidae,  199 

Pelvic  fins,  100 

Pelvic  girdle,  of  Squalus,  114;  of 
Teleostomi,  127;  of  turtle,  228, 
229* 

Peragale  lagotis,  355  *,  356 

Peramys,  354 

Perameles,  356,  embryo  of,  405,  406  * 

Peramelidse,  356 

Perch,  111,  146 

Percosoces,  111,  145 

Perennibranchiata,  186 

Perissidactyla,  346,  392-394 

Permian  reptiles,  214-217 

Petaurus,  357 

Petrels,  294  ' 

Petrogale  xanthopus,  358  *,  359 

Petromyzon,  93  *;  P.  wilderi,  95  *;  egg 
of  P.  marinus,  164 

Petromyzontia,  91-96 

Phcenicopterus,  beak  of,  282  * 

Phaeton,  foot  of,  280  * 

Phalanger  maculatus,  358  * 

Phalangeridse,  357 

Phenacodus  primcevus,  342  * 

Phaneroglossa,  196,  198-203 

Pharomacrus  moccino,  307  * 

Pharyngeal  clefts,  definition  of,  31;  in 
Amphioxus,  38;  in  tunicates,  50;  in 
Myxine,  90;  in  Squalus,  114 

Pharynx,  of  Amphioxus,  38;  of  tuni- 
cates 50,  51  * 

Phascolarctus,  357,  358  *;  P.  cinereus, 
358* 

Phascologale,  356 

Phascolomydidae,  359 

Phascolomys,  359 


Phasianus,  foot  of,  280  *;  P.  colchicus, 

303* 

Pheasants,  299,  303  * 
Philander,  354 
Phoca    grcenlandica,     369*,     370,     P. 

vitulina,  370 
PhociaB,  370 

Phocochcerus  cethiopicus,  391  * 
Pholidogaster,  180 
Pholidota,  346,  377 
Phoronidia,  33,  60,  67,  68 
Phoronis,  33,  60,  67,  68 
Phosphorescent  organs,  in  fishes,   18, 

137 

Photostomias  guernei,  136  * 
Phrynosoma  cornutum,  250  *,  251 
Phyllopteryx  eques,  145  * 
Physeler  macrocephalus,  400,  401  *,  402 
Pica,  372 
Pici,  308 
Pickerel,  111 
Picus,  foot  of,  280  * 
Pigeon,  rock,  304;  passenger,  304,  305 
Pike,  142 

Pilosa,  374,  375  *,  376 
Pinnipedia,  368-370 
Pipa  americana,  196,  197  *,  198 
Pipe-fish,  111,  143,  144 
Pisces,  33,  68,  69,  97 
Pithecanthropus    erectus,    387  *,    388  * 
Placenta,  allantoic,  409;  chorionic,  409, 

410 

Placoid  scales,  109,  110;  in  Squalus,  114 
Plagiaulacidae,  340 
Plagiostomi,  111,  121-125 
Plastron,  of  turtle,  226,  227  *,  228 
Platalea,  beak  of,  282  * 
Platurus  laticaudatus,  256  *,  259 
Platypus  (see  Ornithorhynchus} 
Platyrrhini,  378,  381 
Platysternidae,  238 
Platysternum,  238 
Plectognathi,  112,  146 
Plesiosauria,  216,  219 
Pleuracanthidae,  110,  119 
Pleuracanthus  ducheni,  120  *,  121 
Pleurodira,  241 
Pleuronectes  cynoglossus,  147  * 
Pleuropterygii,  110 
Podiceps,  foot  of,  280  * 
Poeciliidae,  142 
Pollock,  146 


INDEX 


429 


Polyembryony,  specific  in  armadillos, 

376,  377 

Polyodon  folium,  132  * 
Polyprotodontia,  345,  354,  355*,  356 
Polypterus,   108,   111,   128,   129,   130*, 

131,  159;  P.  bichir,  129;  P.  senegalus, 

129,  130  * 

Porcupines,  372  *,  373 
Porcupine-fish,  112,  151,  152 
Porpoise,  17  *,  402 
Prsecoces,  323 
Prairie-hen,  299 

Pre-oral  body  cavity,  of  Amphioxus,  48 
-Pre-oral  pit,  of  Amphioxus,  48 
Primates,  346,  378-388;  classification 

of,  378 

Priodontes,  376 
Pristidae,  123,  124  *,  125 
Pristis  antiquorum,  124  * 
Pro-avis    hypothetical    cursorial    an- 
cestor of  birds,  272  * 
Proboscidia,  346,  395  *,  396 
Procellariiformes,  289,  294,  295 
Procyon  lotor,  367  *,  368 
Procyonidse,  368 
Proechidna  bruinjnii,    349  *,    350;    P. 

nigroaculeata,  349  *,  350 
Pro-mammals,    time,    place,    etc.,    of 

origin,  335,  336 
Pronephros,    in    Myxinoidea,    90;    of 

Ammocates,  95 
Proteidae,  192-194 
Proteus  anguineus,  192,  193  * 
Protopoda,  214 
Protopterus  cethiopicus,    112,    155,    158 

P.  annectans,  155  *;  P.  dolloi,  158 
Prototheria,  345,  347-352 
Prosauria,  231,  232 
Psephurus,  132 
Pseudobranchus  striatus,  194 
Pseudosuchia,  242 
Psittacus  erithacus,  303  * 
Psittaci,  305 
Ptarmigan,  299 
Pteraspis,  170 
Pterobranchia,  33,  60 
Pterocles,  302 
Pterodactyl,  218.  224 
Pterodon,  224 

Pteroglossus,  beak  of,  282  * 
Pteropus,  363 
Ptilodus  gracilis,  skull  of,  338  * 


Ptychodera,  64,  84 

Puffer,  112,  151,  152 

Puffin,  302 

Pyrosoma,  56  * . 

Python,  256  *,  P.  seba,  256  * 

Pytonius,  180  * 

Quail,  299 
Quezal,  307  *,  308 

Raccoons,  368,  367  * 

Racial   senescence,   structural  criteria 

of,  20,  21,  163 
Rag-fish,  111 

Raia  batis,  124  *;  eggs  of,  164 
Rana    pipiens,    200  *,    R.    catesbiana, 

200*,   203;   R.   esculenta,   200*;   R. 

temporaria,  203 
Ranidse,  202,  203 
Raninse,  203 
Ranzania  makua,  153  * 
Ratitae  (see  Neornithes  RatitsB) 
Ratite  birds  (see  Neornithes  Ratitae) 
Recapitulation  theory,  23,  24 
Recurvirostra,  foot  of,  280  *;  beak  of, 

282* 

Remora  brachyptera,  148  *,  149 
Reptilia,    13,    14,   69,   210-259;   char- 
acters of,  225,  226 
Respiratory  system,  of  Amphioxus,  38; 

of  Balanoglossus,  64;  of  lamprey,  92; 

of  Pisces,  106  *,  107  *,  108,   109;  of 

Squalus,  116;  of  turtle,  231;  of  bird, 

270 

Rhabdopleura,  31,  32,  65,  66,  67  * 
Rhacophorus  pardalis,  200  *,  201 
Rhamphastus  ariel,  307  * 
Rhea,  284,  285*;  R.  americana,  284, 

285* 

Rheiformes,  284 
Rhina  squatina,  122  *,  123 
Rhinoceros  bicornis,  393  * 
Rhinocerotidse,  393,  394 
Rhinodontidae,  121,  122,  123 
Rhodeus  amarus,  embryos  of,  135  *,  137 
Rhynchocephalia,  216,  232,  233 
Rhynchops,  beak  of,  282  * 
Rhynchotus  rufescens,  303  * 
Rhytidoceros  undulatus,  303  * 
Ribbon-fish,  150 
Road  runner,  305 
Rodentia,  346,  370-373 


430 


INDEX 


Roller,  306;  common,  307  * 

Rorquals,  404 

Ruminantia,  389-392 

Running  birds  (see  Neornithes  Ratitae) 

Salamandra    maculosa,    189,    190;    S. 

atra,  191 

Salamandridse,  188-192 
Salmofario,  139* 
Salmon,  111,  138,  139  * 
Salmonidae,  138,  139 
Salpa,  58 

Salpians  (see  Thaliacea) 
Sand-eel,  111 
Sapsucker,  309 

Sarcophilus  ursinus,  355  *,  356 
Sargassum  fish,  151 
Sauria,  246-259 
Sauripterus    taylori,    pectoral    fin    of, 

178* 

Saurischia,  219 
Sauropoda,  220 
Saw-fish,  111,  123,  125 
Scales,  types  of  in  fishes,  109,  110 
Sceloporus  spinosus,  248  *,  249 
Sciuromorpha,  371,  372 
Sciuropterus  volucella,  372  * 
Schizocardium,  62  *,  63 
Schuchert,  G.,  335 
Scolopax  rustica,  303  * 
Scombridse,  147 
Scorpoenidse,  149 
Screamer,  296;  horned,  296 
Scyttium  canescens,   122;  eggs  of,   164 
Scymnognaihus,  215  *,  216 
Scyphophori,  137 
Sea-squirts  (see  Ascidiacea) 
Sea-horses,  111,  143,  144  *,  145  * 
Seal,  368,  369  *,  370 
Sea-lion,  368,  369  *,  370 
Sea-moth,  144,  145 
Secretary  bird,  298 
Selachii,  111,  121-123 
Semon,  167,  168 
Senescence,  racial,  20;  internal  causes 

of,  22 
Senescent  types  of  vertebrates,  12,  14, 

15 

Serranus,  eggs  of,  164 
Seymouria,  215  *,  216 
Shark,  13,  17  *,  115  * 
Shark-sucker,  111,  148  *?  149 


Sheep,  392 

Shrews,  360,  361  *,  362 

Siluridse,  140 

Siluroid  fish,  140  * 

Simia  satyrus,  382,  383  * 

Simiidse,  378,  381-384 

Simplicidentata,  371 

Siredon  axolotl,  189,  190  * 

Siren  lacertina,  193  * 

Sirenia,  346,  396,  397* 

Sirenidse,  194 

Skate,  123,  124  * 

Skeletal  rods  of    branchial  basket  of 

Amphioxus,  38 
Skeletal    system,  of  Squalus,   114;  of 

Amphibia,   179;  of  turtle,  227-229; 

of  bird,  267,  268* 
Slow  loris,  380 
Smilodon,  370  * 
Sminthopsis,  356 
Snakes    (see   Ophidia) ;   rattle,    256*, 

258;  sea,  256  *,  259;  venom  of,  257; 

coral,  257 
Solenocytes,  41  * 
Sparrow,  English,  312,  313 
Spawning,     of     Amphioxus,     42;     of 

lamprey,    94  *;    of    Amia,    134;    of 

Stickleback,   144;  of  sea-horse,   144 
Specialized  types  of  verbetrates,  13-15 
Specializations  and  adaptations,  15 
Spelerpes  fuscus,   188  *;  S.  bilineatus, 

189 

Spermaceti,  402 
Sphenisciformes,  289,  291,  292 
Sphcnodon,  232,  233  * 
Sphyrna  zygcena,  122  * 
Sphyrnidse,  121,  122 
Spinescence,     a     criterion     of     racial 

senescence,  21 
Spiracles,   of  Squalus,   114;  of  Polyp- 

terus,  131;  of  frog  larva,  207 
Spiral  valve,  of  lampreys,  92,  of  fishes, 

101;  of  Squalus,  115;  of  Polyptcrus, 

131 

Spoon-bill,  132  *;  295,  296 
Squalus  acanthias,  113  *-119 
Squamata,  216,  246-259 
Stapes,  325 
Stegocephali,  180-183 
Stegocephali  Leptospondyli,  181 
Stegocephali  Temnospondyli,  181 
Stegocephali   Stereospondyli,    181-182 


INDEX 


431 


Stegodon,  396,  397  * 

Stegosauria,  221,  222 

Stegosaurus,  222  * 

Stickle-back,  111,  143,  144* 

Sting  rays,  123,  124  * 

Stoasodon  narinari,  124  * 

Stork,  white,  293  *,  295 

Striges,  306 

Struthio,  foot  of,  280  *;  C.  camelus, 
283,  285*;  S.  molybdophanes,  283; 
S.  auslralis,  283 

Struthiomimus,  218  *,  221 

Struthioniformes,  283,  284 

Sturgeons,  111,  132  *,  133 

Sudoriparous  glands,  327 

Suina,  389 

Sun-bitterus,  301 

Sun-fishes,  112,  152,  153;  fresh  water, 
146 

Susceptibility   method    of    demon- 
strating axial  gradient,  10 

Swallows,  311  * 

Swans,  296 

Swifts,  308 

Swim-bladder,  in  fishes,  102 

Symbranchii,  111,  140 

Synapsida,  214 

Synotus  barbastellus,  365  * 

Tseniodonta,  340 

Tamandita,  374 

Tapiridse,  392 

Tapirus,  393  * 

Tarentola  mauritanica,  248  * 

Tarpon,  111,  138* 

Tarsipes,  357 

Tarsius  spectrum,  380 

Tashiro  biometer,  10 

Taxidea  taxus,  367  * 

Teeth,  of  Squalus,  114;  of  dog,  329  *; 
various  types  of,  329  *;  bunodont, 
329;  lophodont,  329;  diphyodont, 
329;  monophyodont,  329;  structure 
of  typical,  329,  330;  development 
of,  330,  331;  of  elephants,  396 

Teleostei,  106,  11},  135-153 

Teleostomi,  106,  107,  111,  127-153; 
characters  of,  127,  128 

Tench,  111 

Terrapene  (see  Cistudo},  238,  239 

Testudinidse,  238-239 

Testudo  groeca,  239;  T.  elephantica,  240  * 

Teirapteryx,  274  * 


Thaliacea,  33,  56,  57  *,  58 

Thecophora,  234-242 

Thelodus,  169  * 

Thinopus  antiquus,  174,  175  *,  179 

Thread-eel,  141 

Thresher  shark,  122  * 

Thylacinidse,  356 

Thylacinus  cynocephalus,  355  *,  356 

Tinamiformes,  289 

Tinamou,  287,  288,  303  * 

Toad,  Surinam,  196,  197*,  198;  fire- 
bellied,  197*,  198;  midwife,  197*, 
198,  199;  spade-foot,  197,  199;  com- 
mon, 197*,  199;  tree,  199,  200*; 
narrow-mouthed,  202;  horned,  250  *, 
251 

Toad-fish,  150 

Tolypeutes,  376 

Toothed  Diving  Birds  (see  Neornithes 
Odontolcse) 

Tornaria  larva  of  Balanoglossus,  64, 84, 
85* 

Torpedinidse,  123 

Torpedo  ocellata,  124  * 

Tortoises  (see  turtles) 

Tosa  fowl,  Japanese,  300 

Toucan,  307,  308,  309 

Trachodon,  221 

Trager,  of  mammal,  408 

Tragulina,  389 

Trematosaurus,  182  * 

Trianops  persicus,  365  * 

Triceratops,  220,  223  * 

Trichechidse,  368 

Trichechus  latirostris,  398  * 

Trichosurus,  ?%7 

Triconodonta,  337,  338  * 

Triconodon  ferox,  jaw  of,  338  * 

Trigger-fish,  151  * 

Trilophodon,  396,  397  * 

Trinacromerion,  218  *,  219 

Trionychidse,  241-242 

Tritemnodon,  341  * 

Triton  tceniatus,  foot  development  of, 
178*;  T,  cristatus,  190*,  191,  192; 
T.  torosus,  192;  T.  virescens,  192 

Trituberculata,  338 

Trogon,  307  *,  308 

Tropic  birds,  295 

Trunk-fish,  112,  151,  152* 

Tuatara  (see  Sphenodon) 

Tubulidentata,  346,  377.  378 


432 


INDEX 


Tunicates  (see  Ascidiacea) 

Tupaia,  336,  360 

Turdus,  foot  of,  280  *;  beak  of,  282  * 

Turkeys,  299 

Turnices,  299 

Turtle,  anatomy  of,  226-231;  external 

characters    of,     226-227,    234-242; 

soft-shelled,    240*,    241;    snapping, 

235*,    236;    pond,    238;    sea,    239; 

land,  239;  snake-necked,  240  *,  241; 

musk,  237 

Tylopoda,  389,  390,  391  * 
Tympanic    membrane,    of    Amphibia, 

179;  of  turtle,  226;  of  alligator,  243 
Types  of  body  form  in  fishes,  97,  98, 

99* 

Typhleps,  258 

Typhlomolge  rathbuni,  193  *,  194 
Tyrannosaurus,  218  *,  219,  220 

Unguiculata,  346,  360-378 

Ungulata,  389-400 

Urochordata,  33,  48,  49  *,  50  *,  51  *, 
52,  53,  54  *,  55  *,  56  *,  57  *,  58,  59,  * 
60;  classification  of,  60;  characteriza- 
tion of,  48,  49 

Urodela,  18&-194 

Urogenital  system,  of  fishes,  102;  of 
Amphibia,  33;  of  turtle,  231;  of 
mammals,  331 

Ursidse,  368 

Ursus,  368,  V.  gyas,  367  * 

Vampires,  364,  365  * 

Varanops,  215  *,  216 

Varanus   albigulares,    250*,    252,    253 

Vertebrae,  of  Cyclostomes,  88;  of 
myxinoids,  90;  of  lampreys,  92;  of 
Squalus,  114;  of  mammals,  326 

Vertebrate  phylogeny,  22-30;  em- 
bryological  aspects  of,  23-25;  geolog- 
ical aspects  of,  26-30 


Vertebrates,  definition  of,  1;  chron- 
ological succession  of,  27  *;  classifi- 
cation of,  33 

Vertebral  theory  of  the  head,  7 

Viverra  civetta,  367  * 

Viverridae,  366 

Vultures,  American,  297,  298;  Old 
World,  298 

Wallace,  A.  R.,  203* 

Walrus,  369  * 

Wapiti,  391  * 

Water  moccasin,  257 

Whalebone,  403  *,  404 

Whales,    400-404;    toothed,    400-403; 

beaked,  402;  sperm,  400,  401  *,  402; 

killer,  401  *;  southern  right,  401  *, 

402;   balleen,  403;  whalebone,  403, 

404;  Greenland  right,  404 
Whale-sharks,  121,  122,  123 
Wheeler,  W.  M.,  19 
Whip-poor-will,  308 
Whip-tailed  rays,  123 
Wiedersheim,  R.,  viii,  103 
Wilder,  H.  H.,  viii,  77,  78,  79,  84,  85, 

189 

Willey,  A.,  35,  42 

Williston,  S.  W.,  182,  214,  215,  216 
Wolf,  timber,  367  * 
Wombat,  359  *,  360 
Woodcock,  American,  302,  303  * 
Woodpeckers,  309 

Xantharpyia,  363,  365 

Yolk-sac,  in  reptiles,  313  *;  in  birds, 
320  *,  321 

Zanclus,    98,     104,     146,     151,     159; 

X.  canescens,  147  * 
Zebra,  392;  Burchell's,  393  * 


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